What year / build # is your car? BDR switched to a beefier axle assy sometime during the RT3 builds. I thought it was during the switch to ventilated rear rotors but may be wrong.
Easy way to tell what you have. Remove the rear hub cap. Peek in and if you have a nut, its the early type. Bolt means newer type.
Early cars RT2 &3 used BMW E36 (325i) axles. Steer clear of the Car-done, Autozone, etc... I use GKN (oem maker for BMW)
early type:
https://www.eeuroparts.com/Parts/161...ar-New-300694/
Backdraft parts:
https://www.iconicparts.com/products...nce-suspension
They sell the old and new axles
Also I'd contact Jay at
Welcome to VINTAGE Motorsports as they are in CT like you and are considered the best BDR (support) dealer.
Jay is also on this site quite a bit and goes by Cashburn.