It looks like it's never too soon to check 5th gear! I had a jerking trailer brake problem last year and it slipped out of 5th at 65mph - once. DC mech drove it and said it was fine but I've been supicious ever since. I only have 32k on mine and am planning a 4k mile trip in June and couldn't stand it any longer. I bought the new nut when it slipped out of gear last year so I was ready! I looked at the nut and it hadn't moved but then I noticed some roughness around the edge of the washer where it contacts the gear. I put the wrench on to see how tight it was and I'd guess it broke loose at well under 200 ft. lbs. The nut, shaft and even the washer were still all tightly bonded together with the locktite but wear between the gear and washer was letting the gear wobble. The gear slid easily off the shaft and had worn into the smooth part of the shaft just enough to catch a fingernail in. The pattern of the gear spline was worn into the washer almost as bad. I don't lug in 5th - I drop to 4th by 1900 rpm. I figured with my usual luck - I better check it out! Time to call Standard Transmission!
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97 3/4Ton, 2WD, 5SP, #11 Plate, AFC kit, 14 Housing, AF942M Air filter, Straight Thru Cat, 4"From Turbo, Airlift, Shortened Frame, Custom Bed, 24CKFS Sunnybrook, HEY FORDBOY POWERSTROKE THIS!
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97 3/4Ton, 2WD, 5SP, #11 Plate, AFC kit, 14 Housing, AF942M Air filter, Straight Thru Cat, 4"From Turbo, Airlift, Shortened Frame, Custom Bed, 24CKFS Sunnybrook, HEY FORDBOY POWERSTROKE THIS!