lift the rear of the car, and put jackstands under the rear axle. then with the trans in 4th gear, slide a prybar through the yoke at the rear of the driveline, (make sure that the prybar is on the differentual side of the rear ujoint), then try to rotate the driveline back and forth.
this will do 2 things at the same time, 1 it will check for slop in the trans, if it's there, you'll hear it. 2 it will allow you to check for slop in the ujoints. if its there, you should be able to see it.
to check backlash in the diff.,set the emergency brake and put the trans into neutral, there should be a small amount of back and forth play, anything more than just a wiggle is too much.
if that does not make your problem show, it could be the spider gears in the diff., to check them put the trans into gear, have one wheel off the ground , the other on the ground and try to rotate the wheel back and forth. that will check for play there. there should be no give at all.
good luck