With the car positioned so that the driveshaft is not putting torque on the diff. grab the input on the diff...driveshaft and see if you can wobble it up and down. More than just a tiny bit of play is not good.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.