Question for you experts...

I want to run a 3.9 gear for track events, and I want to save my stock 3.54 rear end for the top gear, high speed, open road events. I want to be able to just swap out the entire pumpkin instead of having to change the gears every time (a FAR easier job).

I am looking to save some dough, so I was looking into buying a used (i.e., salvaged) rear end. I know that the Base model cars don't have limited slip, so I am looking for a non-Base model rear end.

There is a salvage dealer in NoCal who wants to sell me the rear end from an automatic. He says that since I want to swap out the gear any way, it doesn't matter if its from a MT or auto. Is this true? He says that the only rear end that his info says is "locked" is the automatic. The used automatic rear end is cheaper than the used MT rear ends - why would this be?

Is there any difference between the automatic rear ends and the 6MT limited slip rear ends (other than the gear ratios)?

Thanks.