Get the code off your axle housing and run it against this:
Axle Code/ Model/ Gear Ratio/ Axle/ Spline shaft/
S201-A Mustang 86-93 2.73 8.8 28
S202-A Mustang 90-93 2.73 8.8 28
S203-F Capri '86 3.08 8.8 L 28
S203-F Mustang 86-93 3.08 8.8 28
S205-F Capri '86 3.27 8.8 L 28
S205-F Mustang 86-93 3.27 8.8 28
S215-A Mustang 90-01 2.73 8.8 28
S217-A Mustang 90-01 3.08 8.8 28
S219-A Mustang 90-01 3.27 8.8 28
S264-F Mustang 87-93 3.73 7.5 28
S265-F Mustang 85-93 3.73 7.5 28
S266-D Mustang 87-93 3.73 7.5 28
S267-F Capri '85-86
Mustang 85-93 3.45 7.5 NL 28
S270-F Capri '85-86
Mustang 85-93 3.08 7.5 NL 28
S271-F Capri '85
Mustang 85-93 3.08 7.5 L 28
S272-F Capri '85-86
Mustang 85-93 2.73 7.5 NL 28
S273-D Capri, Mustang '85-86 2.73 7.5 L 28
S274-F Capri '85 2.73 7.5 L 28
S274-F Mustang 85 2.73 7.5 L 28
S275-D Capri, Mustang '85-86 2.73 7.5 NL 28
S276-F Capri '85-86
Mustang 85-93 3.27 7.5 L 28
S277-F Capri '85-86
Mustang 85-93 3.27 7.5 NL 28
S279-D Capri '85-86
Mustang 85-93 3.08 7.5 NL 28
S281-D Capri '85-86
Mustang 85-93 3.45 7.5 NL 28
S282-D Capri '85-86
Mustang 85-93 3.27 7.5 NL 28
S290-A Mustang 90-01 3.45 7.5 NL 28
S292-A Mustang 90-901 3.73 7.5 NL 28