What Differenital??
 

Say you had a 1994 Mustang with an 8.8 rearend. Your set up put out 600-900 Ft/tor. You want to be able to drive your car to get milk of down to the beach. Take your honey out on a date. What differiantal would you put in. You also have 31 spline axles and 3.73 rear end. Thanks.

AWD:D

you're never gonna hook up so i have no problems recommending the 31 spline eaton posi. quiet, smooth, and it works!

To handle that amount of power you should use a ford 9" with a detroit locker
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Try Drive Train Direct, Its where I got my 31s Eaton and axles. Good prices.

the Auburn pro series is a nice piece too, of course, you could use a Zexel Torsen, the best diff I have ever come across.

The torsen is a nice, smooth diff, I love my T2. At the track it's tough to find the spin/bog line with the bog usually winning. around corners though it definitely keeps the car going straight. For the amount of torque you've got though, I wouldn't launch it on slicks.
I'd take a look at ones like the ARB air locker, I think it is a pretty cool setup.

What is better the 31-spline Auburn Pro or the 31-spline Eaton posi?

It depends on the PRIMARY purpose of the car. For a street/strip application the eaton will be great. for a strip only strength will be your primary concern. The auburn is not as good in the twisties/street. I would not hesitate to put slicks on the eaton...unless that's all you're using the car for.