Diffs are generally cheap, the Limited Slip ones are more, usually £150.
The most common are the Sierra 7" ones with 3.6 and 3.9, you can also get 3.38 and 3.14 but very rare.
Cosworth diffs and some Granada ones are 3.6 and 3.9.
All 4x4 cars are LSD but not Granadas with Traction control.
2 ways of telling what ratio you have, the VIN plate will have a code in box 10 maked Axle.
A metal tag on the diff will have letters and numbers 2 or 3 As = LSD the number will have something like 400362 for a 3.6 and 400392 for a 3.9 diff.
You can mark the input flange and the output flange and measure howmany turns you get. i.e one turn of the output = 3.9 input if it is a 3.9 diff.
The only time it is worth changing the ratio is if you have a LSD and want to change the ratio from a non LSD diff, you then have to change both Crown wheel and Pinion as they are matched, all new bearings should be fitted and the end float needs to be set by someone who knows how to do it.
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