Puzzled by my 8.8" LSD

[1998Exp]

Have been hearing some suspicious rumble from the rear axle of my '98 Limited. It's the V8 version with AWD and limited slip differential. The Ford shop manual tells to test the 'breakaway' torque of the clutches by jacking up one wheel and putting the transmission in neutral. According to the manual, the wheel should start turning at about 20 ft-lb applied and continue moving around. When I do that, I only get the wheel to turn a small angle, because the driveshaft won't move. I believe that's because the front wheels are on the floor, and the AWD transfer case attempts to have them turn when the driveshaft is moved.

I also tried the traditional method of checking if the LSD is working by jacking both wheels and spinning one. After applying a bit of torque, the other wheel is turning in the opposite direction, as if it were an open diff. Seems to me that this is again because of the transfer case.

Can anyone shed some light on my confusion? Thanks!!!

 

[1998Exp]

Answering my own question:
Raised one front wheel up in the air, and everything works as expected. To test the LSD in an AWD vehicle, you obviously need to let the driveshaft rotate freely -- which means allowing at least one front wheel to spin.
Incidentally, the breakaway torque is about 40 ft-lbs. Guess the LSD clutches are OK.
I also took the cover off the differential, drained the oil and looked inside. Found a few small metal shavings -- I am guessing from wearing clutch material. Otherwise there is just a bit of backlash in the two pinion gears. Could that be the cause of the rumble or is it normal?