There's way too much bad info out there about CB antennas. I don't pretend to know the right answer, but I can say with fairly high certainty that: an antenna is a two-part system. The antenna itself, and the reference plane (ie, your car). If you've got a 4' whip mounted to the back bumper of your Expedition, it's not going to work very well, because the body of the vehicle is going to block the antenna from radiating forward of the vehicle. The antenna is going to work better the higher it is off the ground, the more of it that is clear of the body, and the closer it is to the center of the vehicle. That will help ensure that the radiation pattern is even.
Co-phased antennas can help fix bad radiation problems... but they're a matched pair, so this one is easier to screw up than not.
Most of the "rules of thumb" about cutting your coax to a specific length are bunk. For one thing, the effective wavelength of the coax is affected by the type of coax. RG-6 and RG-59 would have to be cut to different lengths in order to both be 1/4 wave, for example. And it can be affected by damaged cable (kinks and bends), etc.
Get a good antenna. Top loaded or center loaded will generally be a better radiator than bottom loaded (a bottom loaded antenna mounted on your hood has most all of the effective length of the antenna mounted at the hood line). Getting it mounted near the center of the vehicle will help ensure that the radiation pattern is even. And checking the SWR and altering the antenna length accordingly will ensure you're getting the best results out of the equipment you have.
Oh, and yeah... it's better if you ground the antenna mount to the body. It can ground itself back through the coax, through the radio, etc... or even through the mount, the toolbox, through the shiny bare-metal spot the toolbox hold-down rubbed on the bed, etc.... but you're guaranteed a good connection if you run a ground wire from the mount directly to the body.
