I hate when I find old threads that were never answered when I am looking for the same answer, so won't leave you hanging:

I found the issue tonight doing some testing on another truck. Truck showed normal mode, running or KOEO (Key on engine off), and could be toggled to TH mode. I went out and unplugged the MAF sensor (which is deleted on the truck that was having an issue), and even without a check engine light it put it right into the TSTATE17-SPLIMP "Engine in Default Mode Shift Pattern" and TH button went unresponsive (light works, but nothing on TCM changes). Plug it back in and it goes straight back to normal or TH mode. I'll be wiring a MAF sensor up in the morning and hopefully at the track tomorrow night taking the record for the fastest stock bottom end back.