This photo (lower right) shows the 5th gear remoxved. We are holding the gear next to where it rides on the main shaft. Notice that the splines the gear ride on only make contact with the gear near the outer portion of the gear. Less than half of the gear actually rides on the splines. When 5th gear is used under heavy loads such as towing or hard or abrupt acceleration, the gear receives pressure, pushing it backward, against the washer and retaining nut. The gear shouldn't move under the pressure, but because the splines on the main shaft don't support the entire length of the gear, it allows the gear to rock back and forth as you apply and release torque on the drive train through acceleration and deceleration. This rocking motion works and pushes against the washer and the retaining nut at the back of gear. Sooner or later, the nut begins to loosen and back off of the threaded shaft. You'll usually feel or hear a clicking in the transmission as the nut backs off over time and the gear begins to slop around on the shaft. When the nut removes itself completely, 5th gear will slide back with it and disengage. If you are on the throttle or the transmission is under a load, the gears will grind as it slides back, teeth will break off and drop into the circulating transmission fluid below. The bits of metal will then proceed with the fluid throughout the lubricated gears of the transmission and lead to more damage. Pretty sweet, huh? The bright side is that if you really needed to, you could get home using your remaining 4 forward gears and reverse. Although, the less driving you do in this situation, the better, because you will damage other gears as the bits of 5th travel through the transmission.