rear of truck makes a low tone vibrating sound?

[texs_22_guy]

info on the ruck its a 94 toyota 4x4 manual 5 speed. at 40+ mph it makes this low tone vibrating sound that will stop if i let off the gas with my clutch is out. if i let of the gas and have the clutch in it makes the sound. all the sound seems to be coming from the rear of the truck but its hard to get out and listen to it at 40+ mph.
thanks brent

 

[WickedMickey]

Your U-joints are going.