I think I'm going to try and make my own lip type mount
Excellent. Start somewhere and see how it works out.
As for the ground, do you think if I ran a wire from the bottom of the metal bracket that I fabricate and attach it to the metal tailgate, or possibly two wires from the bracket to two separate spots on the tailgate, do yu think that would work as a good ground?
I'll use my own post to answer that:
Just "grounding" an antenna to the car body does not automatically satisfy the requirement of a ground plane
You can try it. The worst that can happen is it won't work well. But it may work well enough for what you're trying to do.