Is this a street car? If so, I wouldn't recommend a spool. You can get a diff from a late model F-150, ranger, or exploder, as some of them used a 31 spline trac-loc. Beware though that the outside diameter of the diff is the same, so since the inside is bigger for the larger axles, you'll have less clutch area so the diff won't grip as good. This will also negate the C-clip eliminator kit as well.