TPS might be the issue if the engine ran like crap too.
My truck has the 47RE and got a crash course in them this year.
The majority of shift determination in the RE is internal sensors and solenoids or by the TCM.
Clogged passages or sticky/leaking solenoids will delay shifts and make the engagement intermittant. The TCM controls shift points and TC lock up. So strongly in fact that if you have two wires shorting between the TCM and the tranny the TC will lock up at 0 miles an hour and stall the vehicle if you stop (guess how I know that
The shift bands are self adjusting but give no indication they have run out of adjustment until one day they just don't shift at all, no slipping, just no shifting (that happened last year).
If it is that intermittent check all the connections and wiring between the computer and the transmission. Electrical gremlins are the most annoying.