If you lift less than or equal to 4", feel free, go for it, and not worry about it. If, you lift 4.01" and above, here is the law regarding such lifts:

The following is information obtained from the New Jersey Department of Motor Vehicles which was submitted by loyal viewers of CustomTruckShowcase.com. Updated on 06.29.00.
Law guidelines and regulations are as follows:

An examination of your vehicle indicates that it has been raised more than 4 inches beyond its manufactured height. Under the provisions of N.J.A.C. 13:20-37.3, such a vehicle must undergo a geometric stability test.

And this test is fairly stringent, BTW. If you do a hack job, expect to have your truck taken off the road (and towed from the inspection facility).

A comprehensive examination of the steering and suspension will be conducted. There is no charge for this inspection. Please call the location of your choice as soon as possible, since the test must be completed prior to the expiratio date of your inspection sticker.

Conditions for the motor vehicles with elevated chassis heights are, and I've added my comments after the [fjb]:

(1) Steering gear ratios and steering wheel left stop to right stop turns must be within 1/2 turn of the original manufacturers specifications.

[fjb] don't screw with steering ratios, they do this this thoroughly. Once you pass the first special inspection, you can make changes from there as its not tested again, however there is a slight possibility if you cause an accident, or are part of an accident where it is determined your truck being lifted aggrevated the severity of the accident, this very well might be tested.

(2) Headlights shall be mounted no lower than 22" nor more than 54" above the ground.

(3) Taillights shall be mounted no lower than 15" nor more than 72" above the ground.

(4) Off road lights must be covered while operating on public roads.

[fjb] Tinted [pass through] lights do not count. You must have full opaque covers, and no light must pass through the covers when the lights are lit during the testing. Slip over "KC" covers are fine, as are hard flip-down covers.

(5) License plates must be mounted no less than 12" nor more than 48" above the ground.

(6) Brake lines and hoses must be protected from excess heat and vibration. They must be mounted in a manner to prevent chafing, undue wear, stress, or unintentional disconnection.

[fjb] common sense, but you should see some of the crap people do to their brake lines trying to extend them for the axles that are now further away.

(7) Vehicles originally equipped with bumpers must have them securely fastened, no less then 16" from the ground to the bottom of the bumper, and shall extend the full width of the vehicle.

(8) Maximum tire diameter allowed will be 38" or 6" over stock, whichever is less.

[fjb] this particular thing depends more on the individual inspector, even though the law is very specific. I have a friend that got 44" tires through inspection no problem on his second try. The first inspection he was failed because the inspector held him to the letter of the law - the second inspector didn't care about tire size, but wanted the license plate lower. So my friend had to take the rear plate off the bumper, and mount the top holes on the bottom plate holes on the bumper, to pass, which he did while the inspector was looking over the rest of the truck.

(9) All tires must be the same size. Maximum tire pressure as stated on tire is recommended. Front tires must be at least 60% of the thread width of the rear tires.

[fjb] This applies to 4WD where it matters, and 2wd where it really doesn't matter.

(10) Front fenders must extend over the full width of the tire thread. Rear tires that are not covered by the fender or other body parts must be equipped with metal protectors or flexible flaps that prevent dirt, debris and water from being thrown.

(11) Exposed fuel tanks must be encased. Side mounted tanks must have a protective bar to prevent rupture.

[fjb] this is because the tanks are now exposed to side and rear impacts, because cars can go underneath the truck more and nail the tanks directly, rather than hitting the tanks if and only if the frame (or rear bumper) is bent severely enough. On highly lifted trucks, the inspectors seem to prefer poly tanks over steel, and will not require you to do so, but will make comments regarding that if you have steel tanks.

(12) Exposed exhaust system components and moving parts must be shielded.

[fjb] while the letter of the law is "exhaust system" they are really focused on the catalytic converter.

(13) Ballast must be securely mounted.

(14) Releasing the steering wheel in a sharp turn shall result in an increase in turning radius.

[fjb] what they are testing for here is auto-centering of the front wheels if you let go. Avid off-roaders (and serious track cars) often have zero (or close to it) toe in/out in so the front wheels don't return to center when you let go of the wheel, making it easier to turn sharply on the track. They indicate here they want the traditional slight toe-out to force zero centering.

The amount of return-to-zero depends on the inspector. Some want it "dead nuts straight" some just want it to yank back towards center

(15) Spacers to increase track width are prohibited.

[fjb] this is refering to wheel spacers. have a set of wheel spacers anywhere you're trailering or towing your truck home - period.

(16) Maximum lift heights for GVWR 4500 lbs and under... 7" above original vehicle height.

(17) Maximum lift heights for GVWR 4501 - 7500 lbs... 9" above original vehicle height.

(18) Maximum lift heights for GVWR 7501 - 10,000 lbs... 11" above original vehicle height.

(19) No blocks are allowed on the front suspension.

[fjb] do this and you're trailer/towing your truck home, after a lecture of what an ignoramous you are.

Like I said above on each line where it applies, different inspectors will torture you less on certain things than other inspectors. I used to have an old early 80's F350 crewcab which passed this inspection (the rules were different 15 years ago, but not drastically so) with 48" meats all around and 21" of lift. Most of the expensive, fabricated stuff passed through just fine, they kept whining that my fender flares weren't wide enough and kept torturing me on the miniscule "hair" of rubber that hung out past the fender flares. See, I didn't just lift, I also used wider rims and wider tires, to gain some stability for turning corners and taking exit ramps (not at insane speeds, just being smart, so I thought). I had 18" of suspension lift and 3" of body lift on that truck.

And BTW, which it looked cool sailing high in the sky, especially with the radical zebra paint... it was incredibly unpleasant to drive on the highway even at slower speeds. It was "too high". I could pull to a traffic light behind a caddy, and not see the hood ornimant of the caddy in front of me. When a light turned green, because I was distracted with conversion, coffee or whatever, lets just say I nailed a few trunk lids with the tires. That's something to keep in mind, as high up as I was, even though I passed the inspection, the bumpers were absolutely useless unless I hit something like a Kenworth and trailer, a school bus, or a box van. Any other kind of impact would have been directly on the side, front or rear of the axles.

But at 20, its one of those things you don't think about or care about.