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Here is what I would do, I have a 98 with a limited slip rear but I think an air locker setup would be better so you could run an open dif when you do not need it locked.
Driving with a locked dif is somtimes a chore especially on lose gravel roads
Does it have to be aftermarket? You asked about a posi but most of these responses are along the lock-in/out lines. How 'bout a LS Ford differential? Unless you need *extreme* performance the TractionLok does pretty well.