I just had my Wavetrac rear diff installed at RRT racing in VA, Dan at Diffs online recommended them. Great BMW specific shop.
From their site: RRT is the leading automotive service and performance center in the Northern Virginia. We have been specializing in BMW’s since our inception and have recently taken on Porsche and Mercedes marques as well. Our service department backs all their work with a competitive warranty and we recommend OEM parts. We offer our clients more personal and higher level service than any dealer can. Our performance and race shop has years of experience building both professional and club level race cars. We perform almost every service in house to ensure the highest quality control standards possible. We have all the capabilities that any manufacturer does in terms of both tools and diagnostics, but being an premier independent shop, we can utilize those much more effectively than the red tape of the dealership allows. Come by and take a tour and meet our staff. You won’t be disappointed.
https://www.rrtautomotive.com/