I'm going through something similar on my 1993 4T60E after having had a transaxle rebuild by an incompetent shop.
When I finally picked the car up, after like two months of it being in his shop, he mentioned something about having problems with using the wrong trans wiring harness, and how it required a 1992 harness.
But I assumed at that time, like two years ago, that he actually resolved that problem.

However, ever since, the trans hasn't shifted right. It's like the torque converter is always on. Bogs down. Low gears slip...
Long story made shorter:

The PWM TCC solenoid is supposed to be grounded by the PCM to engage it, and it varies the fluid pressure on the TCC solenoid line (or something)... but, in my case, there is no 12V signal on that wire to be grounded -- which is the root of the problems I'm having. I think it's stuck fully engaged (or is it disengaged?).

TunerProRT is telling me that the TCC PWM duty cycle is 0% pretty much all of the time, except at highway speeds where it fluctuates. The factory service manual is telling me that 0% is full line pressure.

So what I think I'm going to have to do is pull the side cover off the transaxle and trace the wiring to the solenoids and look for shorts/opens.