Open the drivers side door. Look down low on the door jamb of the truck about shin high of your right leg. Just below the bar code on that tag it will say "axle"
31 - 3.73 non-limited slip, F-250/Excursion
C1 - 3.73 limited slip, F-250/Excursion
32 - 4.10 non-limited slip, F-250
C2 - 4.10 limited slip, F-250/Excursion
33 - 4.30 non-limited slip, F-250
C3 - 4.30 limited slip, F-250/Excursion
36 - 4.56 non-limited slip, F-250
C6 - 4.56 limited slip, F-250
41 - 3.73 non-limited slip, F-350 (single rear wheels)
D1 - 3.73 limited slip, F-350 (single rear wheels)
42 - 4.10 non-limited slip, F-350 (single rear wheels)
D2 - 4.10 limited slip, F-350 (single rear wheels)
43 - 4.30 non-limited slip, F-350 (single rear wheels)
D3 - 4.30 limited slip, F-350 (single rear wheels)
46 - 4.56 non-limited slip, F-350 (single rear wheels)
D6 - 4.56 limited slip, F-350 (single rear wheels)
F1 - 3.73 limited slip, F-350 (dual rear wheels)
62 - 4.10 non-limited slip, F-350 (dual rear wheels)
63 - 4.30 non-limited slip, F-350 (dual rear wheels)
66 - 4.56 non-limited slip, F-350 (dual rear wheels)
F6 - 4.56 limited slip, F-350 (dual rear wheels)
81 - 3.73 non-limited slip, F-350 Chassis Cab, narrow frame (dual rear wheels)
E1 - 3.73 limited slip, F-350 Chassis Cab, narrow frame (dual rear wheels)
82 - 4.10 non-limited slip, F-350 Chassis Cab, narrow frame (dual rear wheels)
E2 - 4.10 limited slip, F-350 Chassis Cab, narrow frame (dual rear wheels)
83 - 4.30 non-limited slip, F-350 Chassis Cab, narrow frame (dual rear wheels)
E3 - 4.30 limited slip, F-350 Chassis Cab, narrow frame (dual rear wheels)
86 - 4.56 non-limited slip, F-350 Chassis Cab, narrow frame (dual rear wheels)
E6 - 4.56 limited slip, F-350 Chassis Cab, narrow frame (dual rear wheels)
88 - 4.88 non-limited slip, F-350 Chassis Cab, narrow frame (dual rear wheels)
EW - 4.88 limited slip, F-350 Chassis Cab, narrow frame (dual rear wheels)
G3 - 4.30 limited slip, F-450 Chassis Cab
73 - 4.30 non-limited slip, F-450 Chassis Cab
78 - 4.88 non-limited slip, F-450 Chassis Cab
G8 - 4.88 limited slip, F-450 Chassis Cab
75 - 5.38 non-limited slip, F-450 Chassis Cab
G5 - 5.38 limited slip, F-450 Chassis Cab
GW - 4.l0 limited slip, F-450 Chassis Cab
98 - 4.88 non-limited slip, F-550 Chassis Cab
K8 - 4.88 limited slip, F-550 Chassis Cab
95 - 5.38 non-limited slip, F-550 Chassis Cab
K5 - 5.38 limited slip, F-550 Chassis Cab
KW - 4.10 limited slip, F-550 Chassis Cab
75 - Dana 80, F53 Recreational Stripped Chassis (Motorhome)
95 - Dana 135, F53 Recreational Stripped Chassis (Motorhome)