Need Help with vibration.

[Premium72]

okay. so around 35 i get this pulsing fibration from the truck. its an all over kinda thing so i dont think its a tire /balancing issue(ive already rotated them with no change n the vibration). i pulled the drive shaft and replaced the center u joint(the rear is new an the front seemed to be in good shape.) the hanger baring isnt all that old and also seems to be in good shape. checkd the dope in the diff, and topped it off just for the heck of it.
also when driving, when the vibration starts and i pop it in to neutral, it stops. but. if i rev the engine to a certian rpms(dont know what no tach installed) the vibration will continue. im not sure if this is in the drvie line(driveshaft to rearend) or in the engine or tranny. i checked all fluids and made sure everything was tight(engine, tranny, drive sahft rear end.) so im at a stant still now. ANY IDEAS??.. thanx. w.