87+ Fox bodie rear ends will fit just fine as long as you put SN95 Axles and brakes on it... The housing and axles tubes are exact same length... very simple swap.... I used a rear out of a turbo coupe for my car so it already had 3:55's...
What SorsCode said is right on. The addition of rear disk brakes in 94 added 3/4" to each side, 1.5" total. If you were to take a fox rear end and bolt it up to the SN-95, the wheels would sit 3/4" further in the wheel well on each side. Of course, this assumes that you had the same backspacing for both the 4-lug and 5-lug wheels.
But if you were to take that fox rear end, and replace the axles and brake assemblies with the SN-95 stuff, it would be identical to the 94+.
BTW, a fox rear end will bolt right up to the 94+ car but I don't *think* the SN-95 brake lines will mate to the Fox axle brake lines (the single rubber line coming from the chassis to the axle housing). It might, but I'm really not too sure.