Pics may help as stu has suggested, but things to check are that everything is fitted and serviceable. Ie thrust race is here and in good condition, Belleville washers are there and not flattened (if they are flat, you will go from a diff that's too loose, to one that's too tight with no real adjustment inbetween), and something else I thought of, are the diff balls the correct size.
Does it feel right but tight if you turn one wheel one way and the other the opposite? Or is it just locked up?
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