Suggested midrange fix for Thiel 2.2 speakers?

Can Thiel rebuild them for you?

Before Thiel starting making their own drivers, they used off the shelf drivers. But.. many of those "off the the shelf" drivers were modified to Jim Thiels specifications. Just using off the shelf driver replacements may cause a mismatch between channels, and/or effect the time and phase response with the cross-overs that were designed around those specific drivers.
If Thiel can't or won't rebuild it (and I really doubt that) perhaps you should try Miller Sound in Philadelphia. If you decide to use someone other than Thiel; I'd suggest sending the matching driver from the other speaker for comparison sake.

http://www.thielaudio.com/
http://www.millersound.net