The most likely culprit is the universal joint. Check it out thoroughly before going to the effort of a transmission swap. It's under the ball-shaped housing at the back end of the transmission. One word of warning- - - -before removing the ball housing retainer at the front of the torque tube, support the rear of the truck with jack stands under the frame rails, not the axle tubes! Weight on the springs will cause the torque tube to slam down as you unbolt the universal joint otherwise. It's also a good idea to support the front of the torque tube with a jack as you disassemble the U-joint, just in case.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!