Rychard;

"When I put the clutch in I get a clunk from the rear of the car" is not really enough info...but if are trying to convey that you are getting the clunk when going from engine loaded and driving drive train, to unloaded, or even deccelerating, with rear wheels driving the engine, you may want to confirm this...

Clunks like this are difficult to locate under static conditions, but as others have posted, this is often caused by things external to diff...sloppy rearend bushings or ujoints...one more thing to check for is slop in drive shaft coupling splines...this can occur in high mileage cars, and will also show up as a clunk during accel to deccel transition...even at parking speeds...I've found that if this is the case, particulates in graphite grease, which don't get as easilly displaced as simple grease, can help to pad the slop...eventually, coupling needs replacement...

Good Hunting