I have a 1994 Silverado Z71 5.7tbi with the 4L60E. The truck will start out in 1st, hard shift into 2nd and 3rd and then I'm stuck in 3rd unless I manually shift into 2nd. When I try to take off it stays in 2nd or 3rd. I can do a reset on ECU and the truck will run and shift fine for an undetermined amount of time then the same problems happen again. So far I have replaced the TPS, Shift A and B Solenoids and checked for codes. No codes have shown. I've cleaned and checked all connections.

 

Sounds similar to a problem that I had with my 1996 Tahoe. When I reset the ECM, the tranny worked fine. Turned out the be the valve body. I did get a CEL and codes whenever the malfunction happened.