Interesting idea. I wouldn't go any shorter than 3'. Definately use a spring and sturdy stud mount. Run a braided ground strap from the gate to the body to ensure a good ground.
Ultimately, the 102" stainless whip coupled with a 6" spring is the best system. This is because 108" is a very close approximation to the 1/4 wavelength of the center of the CB frequency range. Shorter antennas are electrically the same length, but physically shorter. They do this by winding the excess in a coil. Top loaded coil antennas are usually the second best type to run, that's why the top of the antenna needs to be above the roofline. That way the majority of the radiated signal is able to leave the antenna efficiently.
I hafta kinda disagree with the statement that aluminum is not good for a ground. I have my antenna (3' Firestik Firefly) mounted to the tire carrier with a stud, no spring, on the flat part of the carrier in front of the 3rd brake light. Fortunate for me, my wheels have enough offset to allow this mounting position. I have had no problem at all with this setup, I have less than 5 ohms resistance between the mount location and the frame/body.
The only thing I have to do to keep the system running well is occasionally take the stud mount off the carrier and scotchbrite the contacting surfaces. The two dissimilar metals (aluminum and stainless) contacting each other create a galvanic corrosion situation, essentially making a battery. If not kept in check, it will eat the aluminum away. Maintain the area and you will have no problems.
I have also run my 102" whip on the tire carrier at highway speeds with very good results. A spring does me no good with this mounting position, the antenna flexes either at the contact point on the tire or the 3rd brake light.
BTW, my SWR with the 3' is 1.2 at 19, 1.6 at 1 and 40. It's 2.1 flat with the 102" whip.
I think it's a good idea worth trying out. If you try this, keep us updated.
Healthy is merely the slowest possible rate at which one can die.