Desperately need help! Truck stalled, won't start.

Hi. We have a 1994 Dodge Ram 1500, 5.2 automatic. My husband drove 1/10th of a mile, put the truck in part to load something in the back. While loading items, it stalled and will not start. They tried putting gas in the carb, unhooking the MAP sensor, still nothing. He has fire. The only codes are 12, 37,55. (but it's had code 37 for a long time - not sure what 12 is or if it even means anything) The other day he drove 30 miles and it sort of lost power, wouldn't pull itself in O.D. going uphill. Then it stopped acting up on the return trip. Could this be a hint as to the problem? Can anyone help us with any ideas on how to figure out the problem? This is his only transportation to work....what a time for this to happen! Any help will be greatly appreciated! Thank you! Julie