The cheapest is to buy a junkyard eaton and rebuild it. That will run you around $200 ($100 for the junkyard eaton + rebuild supplies)
OR
You can buy a brand new high performance eaton from gears R us or other vendors for around $330.
If you are really cheap, you can buy a right rear air bag and crank up the pressure to 25-30 psi. Your car will sit visibly crooked when viewed from the rear, but you will get better traction.