Is my rearend the problem or something else?

Okay..... I am going to try this again. I have a 91 mazda, standard trans, 2.2 engine. When i bought the truck, it was being used for a parts delivery vehicle, so the back was jacked up and i figured they put different gears in it to make hauling the engines easier. I replaced the rearend with one I got from the wrecking yard, but it still has the problem. When I am shifting, it seems like the trans in always in a lower gear. I KNOW it is shifting through all the gears, but the engine is screaming when i get into the higher gears. Could it be the rearend with the wrong gears or the transmission? OR, anything else?