Yes, you should tune to ch 20. Lowest on 20 is lowest across the spectrum, and should be the bottom of a bell curve, not a line (unless you are tuning for a specific channel).
Grounding an antenna is not like grounding a typical load like a light, it is much more of a bonding. A simple small wire to common ground may be woefully insufficient. A quick test method is to take large gauge jumper cables and clip them to the ant mount and jeep frame, then test again. If your numbers get lower, your bond is not good 'nuff. Usually 2-3 means just that.
As said, try different combinations of removing the locknut or screw to shorten it further than it will now go.
Jeeps have plenty of groundplane-able metal, you just gotta get bonded to it.