[Clyde65]

Your 12 bolt will be a series 4, mean 3.73-4.10 ratios only.

To get the rpms down to non screaming, your going to need to do one of the things you don’t want to do. The 5speed swap would be an awesome swap, but there is more than just the trans swap. Do a search for 5speed swap and look that over.

With your 4 series carrier, 3.73 will be the lowest ratio you can go without swapping the whole rear end housing. So you may consider just finding a 12bolt 3 series with a 3.08 and rebuild with new bearings, seals and 3.42’s as well.

Good luck!