It's probably axle wrap, its a frequent problem on trucks with leaf sprung rear ends. It's caused by the axle wrapping up on the rear springs and then kicking out while decelerating or accelerating. It is a clunk feeling and at first I thought it was the transmission also. It was a problem on my old Toy.
It can be fixed or reduced by getting stiffer springs or traction bars.
http://www.4x4wire.com/jeep/tech/susp/axlewrap/