Replaced the rear meain seal and oil pump on 88 dodge ram

Could be you done some thing to the oil gauge senser, it may be unhooked , or you might have bent the senser, or the gauge might just be bad, do you have oil movement now, like under the cover visible oil flow, if there is it might just be what you think , i dont think it would have made it 3 miles if you had no oil flow, and when a new oil pump is installed it takes a bit for it to pump up the oil, i think you are on the right track,

I would either pull off the valve cover and see if you have oild flow while the truck is running. or go by a new pressure gauge. some parts store have them for around15$ and that can save you a big headache in the long run