just got my truck on the road. after replacing rear wheel bearings/seals. gas tank, and some body work. and i noticed while driving im hearing a pretty loud whining noise coming from under the truck but it's only when i'm on the gas. if i let off its all quiet. is it the rear diff? or maybe the center bearing? i grabbed the drive shaft and gave it a few yanks and it moves around pretty easily

 

The drive shaft moves easily side to side? Sounds like you got some bad U-Joints...

 

it has a light clunk when i get on and off the gas too, also vibrates at lower speeds

 

Definitely U-joints!

 

i checked the u-joints they're fine. if not in gear, the axle moves back and forth like i'm twisting the throttle on a motorcycle, it moves quite a bit until it grabs in the rear i believe. i'm thinking that my rear end is clunking when i take off or get on the gas because of this free movement.

 

Sounds like the pumpkin/third member will need to git pulled and checked/rebuilt then.