So it kinda hit me yesterday. I put a low mile 4L80 in my 98. I'm thinjin 25k miles. Well I was thinjikin reverse took a dump on me. Seemed odd. Then last nite while playin musical trucks on my property I thought about my overlooked "roundtoit" issue. The rear axle will randomly lock the rear tires in reverse. I shift into reverse it engages strong n can feel the truck lurch, but it doesn't wanna move, but once I get it movin it responds great to throttle til the axle locks n skids. It's the original high mileage non-G80 10 bolt. I'd heard that I think was the center pin slides out for one reason or another n jams into housing. I'm starting to think my reverse issue is actually a differential issue. I could be way off but just thinkin thru before I go n rebuild a possibly good 4L80 when I have a good G80 14b SF sittin here, ready to be bolted in. It was a city water parks truck and dude drove it over 2hrs home before we took trans out so that's also part of my reasoning.