That thrust bearing was probably damaged as result of all the metal floating around. Even destroyed its not going to allow the diff to walk around enough to cause your gear failure. The IS2 diff I pictured above had a bearing that was fine, I took it apart and it has some rollers that were worn so I replaced it anyways but I have never heard of that bearing failing from burnouts. I was kinda shocked to see what you had happen given your symptoms as that is not very common. Normall the cross pin and spider gears are junk. Doing one wheel burnouts puts tons of stress on the gears and since our great 7w trans fluid doesnt hold up well under extreme conditions like this the spider gear will actually get so hot on the pin they try to weld themselves together! Normally a bad diff only shows signs of wear on the pin and the two spider gears, everything else in good shape. This is a really big reason I prefer using the fluid blend that I do as it sticks to the parts better under such conditions and I am yet to see a diff failure to date since using my brew in these transmissions. When only one wheel is spinning then the diff spins at TWICE the speed of the speedo! So if the speedo climbs to 40 or 50 mph when doing a burnout with one wheel, the diff is really spinning the equivilant of 80-100 mph, but in addition to that speed the poor little spider gears are spinning fast as well and the ATF just gets flung off and there is no lube oil to prevent the carnage, well not for long anyways!
