An install kit is suggested when you replace your rear gears.  The Ford Racing overhaul kit includes axle seals and axle bearings on top of all the shims, crush washer, pinion nut etc that’s included in the basic install kit.  If the car has over 80k miles then I highly suggest the overhaul kit so you hopefully never have to go in there again.

I am running the Ford Racing overhaul kit in Casper and quality and performance have been fantastic so far!