I have a strange problem, I have a 91 ram w150 long wheel base, 318 v8, 4 speed granny low trans, np208 transfer case. While driving at any speed above 15 to 20 mph, as soon as you let off the gas, the truck starts vibrating bad. If you press the brake while still easing on the gas (keeping tension on the driveline) it will not do it, if you shift to a lower gear to keep the rpm's up it will not do it. The truck shakes so bad that if you let off the gas around 50 to 55, it will jump out of gear from the vibration. I have replaced the slip yoke and both front and rear u-joints, rotated the shaft 180 degrees, and used a laser to make sure the slip yoke was in time with the back of the shaft. I think it may be a bent or unbalanced driveshaft, but I am not sure. Like I said, it only does it when you let off the throttle, as long as it's under power, it is smooth as silk. Does anyone have an ideal????

 

check front shaft ujoints at tcase

 

check front and back driveshafts for sloppiness. that kinda vibration will be noticeable right away. i think that like you say its driveshafts.
i would grab every drive component and jiggle it - start from the wheel and work in.

maybe your wheels are very VERY unbalanced?

anyhow, if it is the driveshaft find it quick or you could end up doing a pole-vault like on Mythbusters!

 

I have not checked the wheel's, but I have checked for play everywhere in the front and rear drivtrain. The new slip yoke on the driveshaft has maybe 1/16th of an inch play. The only thing I can think of is a warped driveshaft. I dont have anyone close to where I live that can check it, does anyone know where to get a new one at? I called all the junkyards and they just laugh when I mention parts for a ram 4x4.

 

If the driveshaft is bent,it is bent no matter what speed you are going or loaded/unloaded.
Sounds like something is loose or worn.
Did this vibration accure before any work was done or after and did it happen all at once?

 

Started after the u-joint by the transfer case threw out.

 

Explain threw out.
Did you reinstall exactly as taken apart?
Front or rear?

 

It lost the cap on the front u-joint rear driveshaft. I replaced both the front and rear u-joints with new ones. The front slip yoke was not the right one for the truck.The cup mounts on the one I had were further apart than the t-case yoke. I went ahead and reinstalled it with a new NAPA u-joint. It started vibrating then. I found the correct slip yoke for the truck brand new, I put it in along with new spicer u-joints fron and rear. It was still vibrating, I read some old post on here and turned the driveshaft 180 degrees. Same thing, I used a laser light to ensure the slip yoke was ligned up the exact same as the rear yoke, still the same. I checked for play in the output bearing, the pinion bearing, the wheel bearings, swapped the front and rear wheels, changed u-joints in the front also. Still the same.

 

Might be something with the front slip yoke,maybe try turning just the front slip yoke 180 to the drive shaft.
What is wrong with the original slip yoke that lost the cap?

 

I will try turning the slip yoke 180, the orginal that was on there took a "bastard" u-joint. The yoke on the t case used the short stubby u-joint, the slip yoke took the longer u-joint. Using the computer at my local parts store, we figured out which slip yoke it was supposed to have. I am going to get another drive shaft tomorrow, the guynI bought the truck from say's he has a bunch and if there is one the right length, I can have it.