Any good place to start in diagnosis??

[Sam Parkinson]

Original owner of 1998 Dodge Ram 2500 Quad cab V-10 4x4 with 116K miles. Recently noticing that when placed down in 4-wheel drive low range the transmission(47RE i think) won't shift into 3rd gear or overdrive, but low and second are ok.....you can even almost run it up into the rpm limiter and it won't go over into third. Put it back in regular 2 wheel drive and the transmission shifts perfectly. Seems to shift ok in 4 wheel drive high range. Tranny fluid level is in the normal range and as far as I can tell it actuates 4 wheel drive as it should. You think this would turn out to be a main computer problem?? thanks for any input.