Transmission Issues -- GM Tranny in 3.0

[zonefive]

Car has 191,000 miles. Noticed that going up steep hills, whole car would shake and I had to put it in manual and shift to higher gear to keep it going.

Tried "Shudder Fix" but it had no effect.

Took car to transmission shop with thought it was the torque converter, but they say that is fine, their eval shows transmission is "losing pressure" under load because of a bad piston somewhere in the tranny itself. Does this make sense?

I authorized a trans removal so the problem can be precisely diagnosed. The shop has a really good reputation and they say they can repair the unit good as new, if the worn parts are limited. Otherwise looking at a used tranny.

Any advice would be appreciated. I know there's lots of threads but they seem to be almost exclusively about the ZF transmissions.