I have a 1995 Wrangler. 5th Gear has been out twice. Both times the same reason. The retaining clip that holds the gear in place breaks and the gear floats down the shaft out of range to engage. It's a $3 part to replace, if you can save the gaskets when you take the tranny apart. However you have to remove the tranny and split the case to get at it. You can buy a kit with all the gaskets and syncronizers for $130 off EBAY, but it doesn't have the retaining clip. You can buy the clip for $3 bucks from a jeep dealer. This should be a recall issue for Jeep. Most the 5 speed trannys on EBay have the same problem. Your problem could be simpler. There's a spring loaded retaining ball that's accessable from the outside of the tranny that holds the shifting forks in place. When they get worn, the gears can spin out to their neutral position.