If it's a whine, it's probably gears and not worth fixing. If it's a rumble, it's probably a bearing and depending on which one(s) it is, it might be worth fixing if you just need to keep it on the road. Axle bearings are easy, carrier bearings are a bit harder, and if the pinion bearings are bad, get another rear. In the long run, go with an 8 3/4 if you need the strength or an 8 1/4 if you just need a bit more and want to change to the big bolt pattern. With a 7 1/4 in there, you're just on borrowed time.....