The shift position indicator does not always show the physical position of the shift lever. If you shift from Park to Reverse, there is no indication of where the shift lever is on the indicator. If I place it in drive, the indicator shows 3, and the transmission will shift 3 gears as you accelerate, but no more. Sometimes you have to shift to 1 to get the transmission to engage and the indicator will show 1. The problem seems to be somewhat random and varies on what the indicator shows compared to the shift lever position.
Got it fixed: Took the truck to Austin Motors in Canon City, Co; they had seen the problem before and ran a scan; what happens is that the two terminals in the shift module on the side of the transmission get hot, and that causes the epoxy in the module to leak into and break contact with the pins. Mostly seen when the truck is hot (it worked perfectly taking it into the garage). They used aftermarket module and harness (saving about half on cost) and said that they had not seen it reoccur. Hope this helps you other drivers of a very fine truck!