Have you been to the L.O.S.T. forum (
http://www.lostkjs.com/forum/phpBB2/). Lots of Liberty owners with info/exp with lifted/nonlifted, pulling trailer, etc.
There you can get an idea to a hitch frame height, but that will also depend on your choice of hitch frame or the integrated rear bumper setup.
I believe the Liberty can pull up to 3000#s un-optioned. With towing options, it could possibly go up to 5000#s. Biggest concern will be the availability of gear ratios to match the tire size to the performance of the lifted Liberty, then to pull a trailer.
Anyways, check over the Liberty specific forum. For a trailer frame, I'd prefer a box outer frame, but if you look at how the older pop-up campers are designed (similar weight and overall box size to a TD), the frame is really light and the floor adds structure to the assembly.