As I recall, the rears are supposed to be attached at roughly a 45 degree angle, not parallel to either the back bumper or the frame rail.
When I installed my JT's, there was no cross-member forward of the rear scissors jacks to attach them to.
I was in a campground and had limited tools and no easy access to get something welded.
So, I bought two 6 foot long "L" shaped pieces of steel from Home Depot. I bolted them together to form a "U" channel. If they had a "U" channel in stock I would have preferred it, but you have to work with what you have.
I bolted the "U" channel to the frame, and then attached the JT's to the U channel per the instructions.
Since I had drilled and tapped my other connections, I did the same here when I attached the U channel to the frame, but then put a nut on the bolt for good measure. It ain't going anywhere.
They have been on for 5 years or more and everything is still cool.