Box of rocks doesn't sound encouraging. I suspect you have a bearing going or a bolt has come loose and a shim or shaft is rubbing somewhere.

If you value this trans, it might be worth tearing down to see if you can find what is going on. If you prefer to find a replacement, I'd just run it till it breaks or gets clearly near breaking. I avoided tearing into these transaxles for a long time until I finally did it. Now I think it is no big deal. If you have the factory manuals, the procedures are kind of out of order but it isn't difficult to disassemble. I'll bet I could have one apart in 20 mins after having done it once or twice.

I think this has made me decide to try and find a donor trans sooner rather than later to try and build up as a replacement. Seems like this is the first big spare item I am going to need to on hand at the race track.

Since you already have an LSD, you could find an open diff spare and transplant the LSD when the time comes. Guard will rebuild your Porsche LSD to better than new specs for ~ $500. I did this last year and it made a world of difference in LSD performance. Braking stability is phenomenal with the rebuilt LSD.

Installation of a diff is probably on the moderate difficulty scale as you do have to set pre-load and ring and pinion backlash using dial indicators. The manuals tell you how to do it but it might not be for everyone.