Rear gears could be either 3.73 or 4.10. It has a 48re transmission, which is the strongest one put in a dodge diesel pickup to that point. I don't know what you mean by shifted strangely-it could be normal for these. They don't shift terribly firmly, in large part due to the too-loose factory torque converter. Or, something could be wrong with it, I dunno. I have good reason to believe that the 48re's are actually MORE reliable than the offerings from ford and gm. A transmission guru on another board was over a bunch of transmission shops, so one day he got bored and pulled the records on how many ford torqshifts and gm allison 1000's they had fixed in diesel pickups, vs how many 48re's. The results were that there were considerably more gm's and fords failing at that time than dodge autos. Please note that this does NOT apply to the earlier dodge transmissions.
Best advice I can give you is to let someone who knows those trucks drive it or at least ride in it to see what they think about the transmission.