You want the antenna to be as long as possible without hitting branches and garage ceilings. The base of the antenna should be no lower than the body of the Jeep.
If you put a 2' antenna mounted on the rear bumper, only a small portion of it will rise above the metal tub. This would not give you very good reception out front but would be fair reception backwards. So, it is all in the position of the antenna and how well it is installed.
I have had good luck with the EVO antenna mount that uses the top corner bolts of the tire carrier. Just make sure to scrape off the paint where the bolts connect. Use some NoOx to keep it from rusting.
FYI, a short well grounded antenna will always work better than a poorly grounded long antenna.