Is it the rear control arm mounts? If it's just this section, they sell frame section caps where you cut out the rotted and welded these frame caps inplace. They come with either just the lower mounts or the longer section with the uppers also. I had both sides of my TJ replaced, but the rest of my frame is in good shape, so was worth the fix. I don't know the full extent of your rot issue, it may be an option. Good luck with fix/swap, I know it sux.