I actually was rough enough on my TW that I broke the sub-frame off. A bolt on one side had rattled loose and the weight of my box and rack back there caused the remaining portion of the frame support to snap. The back fender was all that held everything together.
I disassembled and stripped down the back part of the frame, jigged it together straight, added a reinforcement bar and welded it back together. It seems stronger than it was originally and haven't had any trouble with it since.
As far as interchangeability between different years of TW, I don't know. I have a nice shop and if it is metal, I can usually fix it.
Happy Trails All
Ron in Boise