Recently have gotten a transmission rebuilt for a customer. The code 0787 keeps popping up and the truck is stuck in 3rd gear it seems like when the code is deleted with a scanner the truck shifts fine and good. I've also changed the PCM hoping that would be the problem but the problem is still there

 

The most common repairs for a P0787 are as follows:

New/clean transmission fluid
Replacing shift solenoid
Repairing shift solenoid circuit wiring

 

Can also try this page for more info?

https://www.obd-codes.com/p0787