Allison code list, tq. specs, and other info

[tcat10001]

tcat10001@gmail.com

We have a 2002 Duramax automatic four wheel drive short wheel base 3/4 ton truck that we took to the shop to have a gasket or (seal) replaced with the transmission when we took it out of the shop and brought it home it started not shifting...the shift light would flash but would not go into gear...they reset the codes and it worked for a bit, but then started having problems again...they replaced the harness...shift sensor and I believe a couple other things, but the truck got worse and it don't shift at all know...they gave up on it. I spoke with a friend of mine, she said her truck did the same thing (not leaking transmission oil though) just not shifting and the mechanic said it was the solenoid in the transmission...I spoke to the mechanic that was working on the truck they indicated they did not look at this...any ideas...is this a good spot to start looking for problems, is there a way we can check them?
Thanks in advance for your help...