5th gear is where it all starts, a small bearing whine will turn into a vibration and soon there may be no 5th gear at all. Then you will loose the whole thing, it is the transmission going out. Do you know what year trans you have? The early 5 speeds are known for this and the reason is big tires, 5th gear is the weakest gear and will be the 1st to go. Even with only 33's 5th cannot handle it, I won't even put mine into 5th except on flat level ground, my current trans already has the tell-tale whine of things to come, the best advise for now is don't use 5th at all it will extend the current life of your trans until you can get it rebuilt.