HOKAY! Got the trans out for the 4th time. Looks like the low-reverse band was somehow engaging the planetary. Not sure if it was just not seated on the case pins correctly, or if the servo was stuck partially engaged (doubt that one) or what. But for now, I'm going to totally disassemble everything for cleaning to get out all the brass-laden transmission fluid from the last time. Not cheaping out this time by reusing fluid, even if filtered.