Rear End Question

So last night I was driving up our drive which is covered in ice and I was in drive giving it a little gas and my car slid backwards down the hill a couple feet but it made this horrible noise from what I think was the rear end. It was a grinding noise but more of 2 metal pieces hitting, in other words I dont think it was the gears? Ideas??? I am thinking the posi-limited slip but to be honest I have no clue. If thats waht it is then I have also been thinking about going with a 12 bolt because I will be going to the track more and more. I would like a 12 bolt with 3.91 gears, but that cost like $2,300 plus a shop install fee of both my nuts...

A rear is pretty easy to install but to be honest, without hearing the sound directly there is no way to know for sure what it was. It could have just been the sound of the skidding tires with the road or it could have been a bearing frying itself.