exploded explorer tranny

[rands]

Hi, this is my first post so I hope its in the right place. I have a 96 explorer which just barley turned over 300 000Km last week. It has been a great vehicle for me, I have put over 150 000km on it since I've owned it. In april was my first real expense, I had to have the tranny rebuilt. Now I've put 11 000km on it and the O.D. light started flashing last week, and then the overdrive wouldn't work at all. I went to go home a couple days ago and the transmission shifted into reverse and then grinded some and was locked up. Drive still worked so I drove the mile to my mechanic and was going to let him deal with it. So the next day he told me I would have to drive it to the transmission shop that did the rebuild, when I shifted into drive it ground a bit more. I shifted into 2nd and could take off and as I got some speed I could shift into drive, nothing vibrating or grinding. So yesterday I get the news from the shop that did the rebuild and they told me that they were not going to cover it under warranty because of driver abuse. They clamied that I got it stuck and just rocked it back and forth until the tranny just gave in. They also said it looked like someone put a grenade in the case. The case was split and just a pile of steel inside. Maybe 5 reuseable parts. I'm wondering what kind of mechanical failure could have caused this. I told them that I never got it stuck, but they won't belive me. I'm not sure what to do. I am quite cabable with a wrench, but inside of a transmission I'm lost. Thanks.