One way to mount HF antennas is to use the "slider" mount that mounts at the side (end) of a Reese-style
hitch. Shaped like an "L" it slips in the end of the
hitch and is secured by a "linch" pin. Some of the
hitches are open at the end and allow this "L" mount to slip inside. Usually they are the same size (2"
square tubing) If the end of the mount (secured to the
truck frame on each side), are "blind", or welded, a piece of 2" square tubing can be welded on to accept the "L" bracket. This places the antenna out to the side of the SUV and keeps the antenna from interferring with the tailgate. These mounts are usually associated with screwdriver antennas, but will
easily accept most any HF antenna.
If you have access to a steel supplier that will sell
2" tubing by the piece, you can cut a piece to fit across the back of the truck and weld/bolt it to the
frame. This is how I made my HF mount for my Chevy truck. Drill matching holes in the "L" bracket for
the linch pin and you are set.
I hope this helps if you cannot use a ball mount on your Highlander.