I'd finally like to put closure to this thread. I was on my way to work about a two weeks ago and spotted a Gen I sitting in the local junkyard, so I stopped by at lunch to check it out. The owner was cool and he told me to go have a look. It was a LS model and although the engine was gone, the trans/transfer case and torque converter were still there. We negotiated a deal and the trans was mine for $300 including a 60 day warranty. I went back a few days later and bagged a bunch of other goodies.
This past weekend, I installed the whole assembly in my '91. Changed all the seals, pan gasket/filter, and shift cable while I was there.

I suspect the shift cable had a lot to do with the truck being junked. This is the 3rd cable I've seen that has the outer plastic sheath pulled out of the crimp(on the threaded end). When that happens, the effective cable length changes, and they start shifting very erratically. You'll think your transmission is bad. It ain't fun changing it(you have to remove the valve body), but it's a heckuva lot cheaper than a replacement tranny. Rockauto has the cable: about $50.

With the "new" transmission in place, I fired it up, crossed my fingers, and put it in reverse. Amazingly enough, the truck backed up! When I put it in drive, it went forward. It's pretty neat how that works.

Very long story, short: Pay very close attention where that crankshaft/flexplate spacer goes. Put it on the wrong side and it will toast your transmission and your torque converter. It goes in between the flexplate and the crankshaft.

Thanks for all your help and support. I'm back on the road!

-Jim