My 75 f250 highboy, 4 spd manual with a 429 CJ vibrates like crazy when locked in 4 wheel. It starts vibrating hard around 25 mph but when you let off it wont quit when you get back down to almost 0. My axle joints are new but could this be the axle spindle bearing? (might not be proper term....but the needle bearings inside the spindle?) or could this be the angle my driveshaft forms to my front end? (the u-joints are new as well) Xfer case is NP205?? its the manual engage with 4L all the way forward, then Neutral, then 2h then 4h.... I dont know if these problems are related, but it also seems to make some noise in the manual tranny in 4 wheel....some clunking and such. I appreciate your ideas.

 

Sounds like driveshaft angle to me. The bearing should make some noise but shouldn't cause such a strong vibration. It could also be that the front drive shaft is bent slightly or in some other way out of balance.

It only vibrates when in 4wd? And if so how long has it been doing this?

 

this is just a thought... have you checked your transfer case mounts lately? that or maybe make sure you engine mounts are still good and tight. just some ideas.

 

Just checking, are you in loose dirt/gravel etc when you are driving 25MPH in 4WD?

 

My only other thought was if the gears are mismatched between front and rear axles. I'm wondering if there's a binding that starts to show around 25. This is why I wondered if the problem had just started or if it's always been this way. I can drive my truck at 60 on the freeway in 4wd with no problems. As long as the gears are matched it shouldn't be a problem.

 

I know for a fact I have a slight angle on my driveshaft going to my front end. I was considering doing a lift of sorts to the truck over the winter here but i am entertaining many ideas, with that in mind...is the best way to fix that angle issue just some sort of shim? what is the best way to check my gear ratios in both ends because i know for a fact the previous owner switched out the front end due to the stock unit failing. I am thinking this redneck dude (nothing wrong with that) didn't even think about matching ratios and put whatever he could find in there. Any ideas on this are greatly appreciated as i am a new highboy fanatic thirsty for knowledge! -Just got the thing and i am in love.

 

There are a couple things you could try to check the ratios. Pull off your diff covers and look at the ring gear. Often times the ring gear will have the ratio stamped somewhere on the side of it so you may be able to see this as you turn it. If it's not stamped you'll want to count the splines on the ring gear and then the splines on the pinion gear. Divide it and that will be your ratio. You can get a rough idea with the wheel turn vs. pinion turn test but we really want to know the exact ratios to be sure they're the same.

You have 2 options for correcting improper angles on the driveshaft. Either the pinion needs to be pointed directly at the transfer case (pinion to driveshaft angle = 0*) or the angle between the pinion to the driveshaft must be identical to the angle between the driveshaft and the transfer case. It's usually easier to put you pinion at a 0* angle though and this can be achieved using angle wedges or shims.

 

Even if it's got the factory gearset in both front and rear differentials, the ratios are not the same. If the rear ratio is 4.10, then the front is 4.09. Don't ask me why Ford did this because I don't know. If your rig has 3.54s in the rear then the front is the same (unless it's been changed).

In all truthfulness, you shouldn't use 4x4 on a surface that doesn't allow slippage anyways. I'm willing to bet you've got a driveshaft angle/balance/bent problem. Those highboys with the divorced t-cases and low pinion dana 44s are weird ducks as far as driveshaft angle goes. If you noticed, the front pinion actually points at the ground and has quite the angle to the u-joint already. It's not the same as the angle on the transfer case as you can see with your eyes.

Cody

 

well just an update on the issue- my "buddy" at our local 4wd specialty shop was slightly puzzled when i took him for a cruise. he said mismatched gears would leave the truck binding up and not moving after about 20 ft of travel, said that driveshaft vibration issues would show up with hubs locked and 2wd engaged on transfer case, and they dont. so being that the issue was not as big as some other things with the truck, we left it at "well...maybe someday when i pull it all apart something will make sense". so i drove on.... Last week i put the truck to the test again...hoping some magic had occured....and it did. The truck still makes a very slight vibration right around 20-25 mph..then disappeares once more speed is acheived. i went 60mph down a dirt road with no issues. i am thinking something might have just been a little bound up because this front end was just put on the truck from a parts vehicle that was sitting for a long time with no gear oil....hmmmm? a little corrosion/rust (scary to think about) could have caused all that ruckus? who knows..... but i am happy it is working up to some sort of reasonable standard before the michigan winter sets in. I also just added a new 750 edlebrock 4bbl and an MSD billet distributor and 6AL power box to the mix, some new lockouts, rancho 5000's and lets not forget the new coil and msd helicore 8.5mm wires. wow did that make a difference over the old points and condensor vacum advance distrib. yippppeee!