I couldn't get the truck to go over 40 miles an hour without going downhill. Uphill even worse.

It's like the truck didn't want to change gears. Now my girlfriend had been to the shop before, and they said her rear differential was leaking and need service. For some reason I keep think slip dif which I thought was no big deal, but after reading some articles online I thought maybe this could be the problem. Do you think the rear dif would cause the truck to act like that, and if so how might I go about repairing it. We don't have much money, and this is the last vehicle I'm down to.