Tayto, you were exactly right. Jumping the OSS wires that came with the US Shift controller kit over to the old OSS wires that I had hanging zipped tied out of the way near the transmission allowed the speedo to work again. That, and the fact that error codes the US Shift controller were throwing went away after a period of power disconnection means the transmission is now shifting normally again. There are probably about 9 things I would have done differently in hindsight. The first would've been to really drill down on the differences on the 88-91 wiring vs later models, as that led to a whole bunch of confusion initially on where the drac is located. Second I would not have assumed the US Shift technical advisor could read a schematic or really understood the product the company he works for sells. Nor would I assume they'd understand much about this particular vehicle. Third, in purchasing the controller I should not have assumed they'd have a pre-printed ready to go, wire for wire installation guide for my vehicle. That's fairly common nowadays, but not the case with US Shift. I realized now they are much more geared to those installing a 4l80E in a pre-electronic control vehicle or hotrod. They just either don't get the request much to do what I needed the controller to do, or they just don't see it as worth the investment in time to figure it out in advance. Either way, good solid tech support would've solved that, and in that regard they're sorely lacking. Fourth, and this really goes back to the initial diagnosis by two separate transmission shops that were hopelessly lackluster, one should never assume that shops that have been in business for decades would necessarily have seen or have the institutional memory to correctly diagnosis what the problem actually was. Which, drum roll please, probably lies in the original wiring harness. Not the drac as one shop was sure of. Not the PCM, which another was too. I actually hunted high and low for a new external wiring harness to run from the transmission up to the speedo and PCM. I simply couldn't find anybody who sold one. Having one custom made was more pricey than the new US Shift controller which came with a wiring harness, so in the end that's the route I took. But cripes, what a saga.
Thanks Tayto, who clearly has a solid understanding of these transmissions and their electronic control. Thanks to others who chimed in, there was some good advice there too. It's all appreciated.