I'm looking for some advice regarding an on-going issue I've had with the rear differential on this vehicle. I recently had the rear differential replaced by my mechanic. Supposedly the case on that one had a "pin-hole" in it, went dry and eventually failed. The mechanic thankfully replaced it his cost - but not before trying to insinuate that it must have been something I did. It'd been driving fine until - A few days back, after having the truck idling in PARK for about 15 minutes or so, I shifted gears into DRIVE, moved forward about 5 feet when the wheels locked up - almost as if it was slipping back into PARK on its own. It would drive in REVERSE just fine. After jagging around for a while with putting it into REVERSE, PARK, back into DRIVE, etc. with no luck, I slipped it into DRIVE, gave it some gas and it "unlocked" so I drove off. A few days later on the freeway I noticed a high whining sound that eventually grew louder. At the time I thought that it seemed to correlate with the gas pedal - grew louder when I accelerated, quieter when I backed off the gas a bit. Anyways I turned around and took the truck back to my mechanic. He found that something inside the differential had ripped through the case, cutting a long, slot-like hole all the way through. He's said that he's never seen anything like it before - insinuating that I must have been driving the truck recklessly somehow. I'm guessing that when I "unlocked" the truck by giving it gas, that something inside the mis-aligned or otherwise improperly-installed differential actually tore up. My question is: What was "locked up" to begin with? (It seems to me that it was an issue with the installation.) and Should he be replacing the part (yet again) at his cost? This is the second differential installed by this mechanic that has in some way failed.