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The normal problem causing this is a part inside the transmission called the conductor plate. It's a part that enables the transmission control unit to gather rpm data and also control the electronic shift valves. But they should have some codes associated with this failure, usually the problem is with one of the rpm sensors and they will have codes for one of these sensors.
The statement about if the truck could have inadvertantly been shifted into low range makes no sense; first, the truck has to be put in to neutral to do this, secondly the truck would still upshift in low range. I wonder if they had a fault code for the low-range motor. There is a postion indicator inside the low range motor, and if the position indicator would fail it may put the truck into limp-home mode. I have replaced a number of low-range motors as well, but my first suspect would be the conductor plate.
I'd be suspicious about the "no code" observation.
Gilly
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