It seems from your posts, that you are set on rebuilding the trans, but before you do that, get some troubleshooting done.
While you had it in the shop, did they change out the Trans fluid/filter?
Have you tried to manually shift the trans.
If you only have 1st and 2nd gear, manual says to check the 2-3 solenoid, here are the problems.
-stuck off
-loose connector
-no volt to solen
-solen o ring failure
-no pcm signal to solen
There are shift solenoids, I would check the 2-3 shift solenoid, red and yellow wires, resistance should be between 20-40 ohms
The very least, I would drop the pan, and check the electrical connectors on the 2-3 shift solendoid
Or you could buy a 2-3 shift solenoid, they are about $25 and change it yourself
Is your check engine light on, if so, what codes are displayed?
How many miles on the trans?