It could be a ton of different things. First off, how many miles on the truck? And it does have the 8.8" rear end, Correct? If so i'd pull your rear diff cover off and slowly spin the drive shaft by hand, Bolted to the carrier there is a speed sensing ring which runs the abs and your speedometer(your cruise runs off your speedo). The 8.8 rear end that you most likely has a tendency to go out. What can happen is that if metal pieces start breaking up and getting at that speed sensing ring it'll get the little teeth bent which sends a bad signal to your speedo, which in turn screws with both your abs and cruise. And it'll also send that screwed up signal to your tranny and cause shift problems. I know this cause what your describing happened to me about two monthes ago. I threw a speed sensor in, checked all the wiring, and almost ordered a gauge cluster and computer before my rear end locked up,,,,i figured it out when that happened. My spider gears were completely ground to nothing. As far as i could figure a piece from one of the clutch packs in the Trak-loc busted off and lodged itself between the sensor ring and the housing. One tooth was bent about a 1/16 of an inch different than the rest, but it was enough to set it off. The sensor ring was really cheap at the dealership, it was only like 20 bucks. Unfortunetly if this is the problem most likely you'll be looking at 50-600 dollars for a complete rear end rebuild. So the first thing i'd try would be to drain the rear end and check that. And even if thats not what it is, changing your diff fluid can't hurt. You can try to change that sensor on top of the housing, its about 15 bucks. Hope its the later. If you have any questions feel free to email me at Ryan50hrl(No Email Addresses In Posts!) If you do end up needing parts i have a bunch of stuff that might work for you. Gook luck