Where is the TCM of a Chevy Express 3500 located?

Hello Dave, The issue is when I drive the van past the 3 gear it doesn’t switch to the 4th. The diagnostic reads as follows: shift solenid B, shift solenoid A torque converter clutch pressure control. But all issues above has been replaced. All cables/harness. And it still reads the same. Which led me to believe it was the TCM. Thank you.

Thank you for your first reply to my question.