Well, I'll get right to the point.
After adding 1.4 L of Oreiley's best gear oil to my previously drained differential, I secured the foot brake with a piece of wood, chocked the front wheels and reversed the ramps, with stop bumps forward.
Confident the car was not going anywhere, neighbor Scott stood alongside while I selected reverse, removed any driveline slack and put the transmission in first. I throttled up to about 2k and quickly engaged the clutch, disengaging just before stall. Same thing in reverse. And first. And reverse. And first.
Nothing from the rear. Zip. Nada. Zilch.
5 more minutes of repeating the process, with Scott as far under as he dared, and still no clank, or anything remarkable.
Off the ramps and onto the street.
Clunk is gone and trip to a local dirt patch confirmed LSD is fully functional.
In my feeble defense, the last time I R&R'ed diff fluid (and thanks to everyone who resisted making that obvious suggestion) I topped off till oil ran out the fill hole...and no apparent diff leaks since. Gotta say, I didn't measure what came out last week.
Waaaait a minute.
Last summer, when my rear pads began to howl, along with the left pad backing plate contacting the rotor, I found the axle seal to be defective and...wait for it...there was diff fluid on everything.
I'll say no more. Duh...
Lessons learned: Go for the low hanging fruit and assume nothing.
And yes, this has been difficult to admit...