So my initial transmission problem was that it had really jerky shifts from 1-2, and sometimes from 4-2. ..
N E ways, I brought my car to a jack-off-all trades type mechanic friend today to change the motor mounts. It took us a while to find out that you have to either lift the motor out or take out the front end in order to do the swap, but in the meantime we had unbolted most of the motor mount brackets, taken off the starter, removed the ac pump and moved some other stuff.
After we put it all back together my transmission problems got weirdly different. Now the 1-2 shift is pretty smooth, delivery seems smoother, the O/D light blinks constantly, and shifts happen much later, at around 4k RPM, even under only moderate throttle.
Also, when I come to a stop, there is a loud, low-pitched humming noise, which ceases after a couple seconds accompanied by a slight lurch forward. There is no shaking or roughness though.