01 jimmy klunk

My 01 Jimmy 4wd SLT has developed a rear end klunk. Service Tech checked and only found a loose axle mount. Tightened it and noise remained. Can any one help out. Usually when in 2hi rear end klunks when it shifts from 1st to 2nd. When I brake and remove the load from the drivetrain and then re-accelerate I hear this klunk. Any guesses?

Check the UJs. A worn diff will do that especially if you're rough on the pedal.