Have a `997 2wd F-150 XLT auto trans with 305,000 miles that has developed at vibration that is most noticeable at 55-60mph. It seems to start at about 35, but at 55 you can feel it in your seat. When it's vibrating shifting into neutral doesn't change anything. Drive shaft u-joints: two rear replaced not too long ago (5+ years?), front joint by transmission seems to move freely, but isn't excessively sloppy, carrier bearing okay. Have done the following with no improvement: re-balanced all tires, no problems noticed during the balance; replaced first right and then left rotor and wheel bearings, no change. Several mechanics have driven it and made different pronouncements, it's not: transmission, rear-end, tires, u-joints or carrier bearing. One mechanic ran it while up on his lift and didn't see a driveshaft balance problem. Last suggestion was that the left wheel bearing were "on their way out", so they were the most recent change. It's drivable, but not fun and it produces a lot of noise in the cab.
Diagnostic suggestions?

 

Alignment? Tires out of round?

 

Alignment? Not recently. Tires out of round or belt shifted? Wouldn't that have shown up during the balancing?

 

Out of round or a broken belt may or may not affect balance. Worth checking....
Have the rear wheel bearings been checked? Pinion bearing? Without experiencing it first hand, I'm just throwing possibilities out there.

 

Climb under there and wiggle your ujoints yourself. Running the shaft while on the lift with no load on them tells you very little, good chance it is one of them.

 

Actually dropped the shaft. All u-joints move without excess slop or binding.

 

since you shift into neutral and drop RPM, then you know its not the motor / converter / transmission input end ... The vibration is relative to ground speed........ transmission output shaft / drive shaft / pinion bearing / differential bearings / axle outboard bearings / tires.
If the tires were spin balanced then that is not it. You checked the drive shaft UJ, but could have thrown a balance weight off ? I think you choices are drive shaft balance , inspection "feel" of outboard axle bearings ........ then all that's left is the differential bearings .... I would think you would get some noise with diff bearings or outboard wheel bearings.

 

Grab the driveshaft and the transmission output end and see if it has any radial shake as in output bearing wear.

 

Swapped front tire(s) with spare one at a time, no change. Mechanic had mentioned some amount of rear axle end play, but didn't know what was normal for a Ford. Another looked over the drive shaft and didn't evidence of any balance weights missing.
Obviously I would like to solve this and at the least cost. Don't want to throw money at it just guessing.
Should I take it to a driveline shop and have the shaft pulled and spun up for balance? Any idea what that costs?
Should I have the mechanic pull the diff cover and axle halves and check for a bad/cracked outer bearing?

 

no one can narrow it down to a specific item over the internet... All we can tell you is bearing problems NORMALLY make some noise also. If you have a vibration, and no noise, it NORMALLY would be tires / driveshaft/ loose mounts on axle - springs.

 

FORD axles are made with approx. 1/16 inch endshake due to the retainer clip design inside the pumpkin.. Clearance in this direction is not a problem.. it is the up and down movement that is a problem and normally will show some oil leakage and have noise with it.

 

How much up and down movement is acceptable?

 

I would say 1/16 inch is ok to run before changing bearings, but have seen up to 1/8 before trouble(leaky seals). Make sure any movement you feel is not just between the axle and the shock eye that can give it a false loose feel. If I were you I would pull the drive shaft and take to a machine shop who specializes in that type of thing. They can diagnose it in 5 minutes if that is the problem.

 

the wheel bearing is a straight roller bearing. The bearing itself has NO stop for in - out movement.. The radial clearance of a bearing is in the .001 inch range... When installed, you can probably feel a few thousands clearance up and down.. That will be due to the shaft / length / spline on the inside end.. The bearing ITSELF has no clearance. Make sure your not feeling the thrust movement when you wiggle up and down. If it seems like it has .010 clearance ( ten thousandths), then you might want to look at it .. Again, you should HEAR something or notice an oil leak if the bearing has excess clearance.

 

Yeah, I get it and I appreciate all the advice and suggestions posted.

Dealing with two different garages and several mechanics that have test driven my truck. Very little consensus in their diagnosis. Once keeps suggesting that the front tires are the cause of all of it which I can't understand, since are about a year old with on 11k miles on them and balance up without a problem. Another says pull the drive shaft anyway... and sent it to a shop to test for balance. Easy to suggest such things when they aren't paying the bill.