Ring and Pinion Backlash

It's quite likely the symptoms you describe are caused by wear problems with the spider gear assembly inside the diff. The thrust washers and plates in the limited slip portion are also worth a close inspection while you're in there. Quite possible the ring and pinion lash is still just fine.

I think you may be correct. With the amount of
"klunk" I can feel if I don't feather the clutch
(centerforce), that would have to be more that
just the ring\pinion. I should just plan to remove ZF, open the lower, and go from there.
I know all the u-joints and axles are good.
Who would sell a good IPC and service\OH manual?

The only known overhaul manual for the DS-25/2 transaxle was published by ZF in Germany (English & German versions)in about'72 and reprinted by Hall, POCA and god-knows-who-all. Copies are available from all the vendors and some Chapters, with a few fuzzy photos (the original was rotogravure-printed and doesn't copy well). There's another manual that covers the DS-25/1 (Mangusta) and parts of the DS-25/zero (GT-40) transaxles. But even with the manual in hand, some sections are unclear, so be warned. The section on checking ring & pinion drag etc is clear, though. All genuine spares come only from RBT Transmissions in L.A. They bought total rights to this trans back in the late '80s. They may also have original manuals.