I've got a '97 4x4 with a 2.7L. I'm hearing a chriping/squeaking noise coming from somewhere in the back end that seems to be proportinal with speed. I'm guessing it's either a u-joint or the rear end. The truck has 80,000 miles and I've never changed the read diff fluid yet, is this really bad? Do people always change their diff fluid? Could it be a bearing? It's very hard to describe the sound.