How can I check Rear Differential?

[jrpmp69]

Is there a way to check a rear differential to see if its good?
The one on my friends 95 talon is going bad and we found a car at the junk yard that has it and we wanted to make sure it was good before bying it.

 

[DSM90AWD]

I'm sure there are other tricks, but for the two used diffs I picked up I checked the following. First I drained the fluid and see if looked watery/rusty which would be bad. Also checked the total backlash via the driveshaft flange: with axles immobilized (e-brake or other) turn the flange as far as possible to the right (and mark on the diff case) and then to the left (and mark). NOw measure the distance between the two marks. On 1Gs, the limit is 5mm but not sure what 2Gs are

 

[viprez586]

Jack up one wheel and turn it. Put a IN/LBS. torque wrench(beam or dial) and check turning torque with OEM specs.