differential prob how do we know?

Just replaced my cross member bushings in the rear, started to get that same feeling... bump,bump bump... almost like a bad cv joint. Checked cv joints, drive-shaft, bushings & everything is tight . Someone suggested it might be the rear diff. it also feels like it bangs into gear

Go to your nearest GM dealer and get a bottle of "Posi-Additive". It's a pint size bottle for about $7-10. Change your differential fluid and add the posi-additive half way through. This may work if you have diff that's kind of bunding up from sitting or running low on fluid. When you change the fluid, don't just use the drain and fill holes. Take off the cover(you'll need a new gasket) and have the car safely lifted off the ground. Then have somebody hand spin the wheels and see if you see any broken parts in there. If not fill it like I said above.
Another thing to check is your universal joint that connects your drive shaft to your rear. It can wear out if you're up in miles of alot of city driving.