i have been have a problem with my 85 2wd 4 speed. when i put it into gear and start moving i hear a clunk. I ease off the gas to change gear i hear another clunk. and when i accelerate the rear sways to the right and when i ease off the gas it sways to the left. is it the u joint or my differntial. i really dont know what it is how could i tell what it is

i dont think a ujoint would cause the truck to veir right or left i lean tward a suspension issue like loose u bolt on axle or the such. check shocks maybe one is binding or a broken leaf spring, look closely and try to shake the whole shaft near the diff flange and see if it moves at all, it should not move in the slighest

In my experience, a loose u-joint makes the truck shake, especially when accelerating at highway speeds. I would think that something is loose on one of your leaf springs, like a bushing or a loose U-bolt. You might need to jack the truck up by the frame to see if the bushings are loose. It might be difficult to wiggle with the weight of the truck on it. Another option is to look at the leaf springs while a friend slowly lets the clutch out in forward and reverse.

Well I just had my driveline u-joints replaced and had it rebalanced so I can definitely say theres a possibility that that is the problem. Initially, I was just getting a clunk, or actually more of a 'clang' then later on down the road I was getting the crazy vibration at certain speeds. You should be able to check to see if its u joint problems by getting underneath and trying to rotate the drive shaft with it in gear. If you can wiggle it around, your u-joints are probably getting worn out.