I am talking twin cams now, not earlier models. There are several options for the rear in the form of Tru Trac, Bagger Brace, etc. but there is a new, cheaper and simpler solution from Sykes Performance called the Drive Train Stabilizer; just out about mid 2014. A simple but elegant solution that, IMHO, solves the problem without linking the tranny to the motor but actually stabilizing the connection between the drive train and the chassis.
I have a set on my bench but have not installed. I can take a picture and post it if interested. Basically it consists of machined hardware that replaces the OEM hardware at the fulcrum point; the swing arm axle. The system takes the flex out of the "hinge" in the frame.
Sykes Performance is selling the stabilizer; early models were ready first and later models should be available now. Like I said up front, don't know if this system retro fits to an early FXR. You can Google Sykes Performance for contact information but the stabilizers are not up on his website yet. Let me know if you want photos and I will post them.:s