" I was guessing on inching as the truck acted like the clutch was pushed in so to speak"

It does act that way, but it is a manually operated hydraulic valve inside the trans that dumps oil pressure from the clutch packs, not an electric switch.

A problem in the seat switch will affect Forward and Reverse alike, but if it is a seat switch issue, the hydraulic functions of the mast will also be inoperative. So, if you have not checked mast function when the trans does not engage, you want to do that to eliminate seat switch as a factor.