Found a place that rebuilds e53 TC and M62 engines
 

He lists rebuilding the transfer case for 950.00 and if you read the text for the e39 M5 S62 engine rebuild he mentions he will rebuild the M62 as well.

BMW X3-5, 3-5series XI T-Case Rebuild Service – Jacks Transmissions

I believe the owner is the guy who had long post on E39 M5 forum. He basically bought M5 and after bringing it home, realized that there was serious issue with the block. The post is full of great pictures and my impression of the guy is the one of great skills and willingness to help others.

I don't have personal experience but I have feeling that he'd be excellent choice for somebody needing new transfer case or something else.