I figured opening up the tranny to replace a synchro would be expensive. As far as the chattering goes its hard to explain. When I am in 5th on the gas it drives fine and as quiet as a 95 jeep can be. When I get off the gas fully, still in 5th gear, and the clutch engaged (not depressed) there is a chatter or rumble right below my center console. It is not so much a griding sound that you get trying to force gears together but more of a rumble that sounds like a gear or something in the tranny is trying to catch but slipping. It is enough to make my truck vibrate a bit (can feel it in your seat) Could it be a bad U-joint?...or would that cause problems in every gear not just 5th?