Question on 96 Silverado Z71 drivetrain

Hey, i'm lookin into buying a 96 silverado Z71, reg cab short bed with a vortec 350, not sure which tranny or rear end. Its a strong running clean truck, but when i was test driving it, i tested its turning radius. Well when i went to back up, i put it in reverse, accelerated, but when i stopped and shifted into drive and gave it gas it made a THUNK sound. And when i was making a turn and accelerated, i heard a slight grinding noise, but i couldn't get it to do it again. Other than that, it drove strong, no slips, no other problems at all. When i asked the guy about it, he showed me a receipt for getting the tranny checked earlier in the week and it checked out good. I'm wondering if it could be something in the rear end, but I'm wondering of any other possible problems? Any ideas?

Let me guess.. you made a right turn when you heard the noise. You almost bet your ass the G80 limited slip carrier is about to snap. Open up rear cover and check the play where the pinion shaft (not pinion gear) goes through the holes in carrier. This is the only way I know to make sure.

Could be normal (so says GM) driveline slop. 4WD vehicles seem to be more prevalent, but the grinding sound seems strange. It would be worthwhile to have an independent mechanic go over it.