I'm been benefiting from the posts here and thought id add my 2 cents. This may have been covered before.
I have a gearbox with no vin, so probably a series 3?
The transfer case also has no vin , so probably a 2A?
I have a series 3 transfer case modified for the Ashcroft high ratio gears
I removed the transfer case and installed the modified case .
The gearbox seized up when i tightened the nut on the mainshaft
Turned out the oil slinger was binding on the new case as the new case has a smaller hole.
I guess the series 3 transfer case has a smaller oil slinger?