Ford Repair: Rear End, mustang gt, rear wheels

Question
I have a 1987 Mustang GT and over the last few days i have been getting a progressively worse clunk in the rear end.  Also a shudder on acceleration.  I have inspected the u-joints and they appear to be fine.  I also took the cover off of the rear end.  When the car is in neutral and i spin one tire, the other sits still.  When the car is in gear and I spin one tire, the other spins the opposite way.  There is a lot more play in the diff. on the passenger side and hardly any on the driver's side when I rotate the wheels individually.  Also, I started the car on the jacks and put it into gear and watched the rear wheels - the driver's side spun but the passenger remained still.  Are these things all normal.  I need to pinpoint my problem.  Could it be an axle? A Differential?  A Bearing?
Any help would be appreciated.
Thank you for your time.
Stewart

Answer
Stewart-

  I'm torn between a differential problem and an axle problem...maybe something like some stripped splines or something.  The only way I can think to know for sure would be to actually drop the rear end and have a look inside.  It could be something as simple as worn out clutches or springs in the differential (depending on what kind of differential you're using).  That said...short of pulling the whole thing apart I'm not sure if you can find the problem one way or the other.  Hope this helps.

Steve