I have heard of cases where the reluctor ring on the differential (final drive) comes loose or shifts on the diff. A simple check is you could remove the VSS sensor from the diff. cover and look inside
the hole and get a visual to make sure the reluctor ring is still in place, and even put a screwdriver on it and make sure it is still secure on the differential (final drive) unit.

If you have access to the equipment your could even put a scope on the VSS singal and compare to a known good patter to verify the signal.