How are you supporting the truck to work under it? If you have jack stands under the axle tubes, the rear end will be forced forward by the weight on the springs. Use stands under the frame and let the rear end hang free. Also, be careful about unbolting anything with the weight of the vehicle on the springs- - - -the torque tube can slam down and injure you if you do manage to get the U joint disconnected.
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!