you can either buy the ring and pinion gears new from just about anywhere,or alot of ford explorers had 3.55 and 3.73 ring and pinion gears that can be swapped into the mustang rear end..you cant use the whole explorer rear end though,no control arm mounting points
Well if you do have a 1985 then you got a 7.5 rear end. Your best bet would be to look for 1986 and up fox so that the swap would be just a drop in. Also like Need said you can find 3.73 in Exploders and Mountaineers.
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