Antennas need to be "resonate" to work properly. Making an antenna resonate at a given frequency is a very specific thing. On the CB radio band, 1/4 wavelength, or about 108 inches, is an ideal length. It will radiate RF energy efficiently.
Of course 108 inches is just too long for most mobile installs. To trick the antenna into being physically shorter and still being resonate, the manufacturer will install loading coils somewhere on the antenna, usually at the base, but sometimes near the middle. These loading coils make the antenna look shorter physically, but also make them long enough electrically to provide a proper match.
Anything under a 1/4 wavelength long is going to be a compromise. Finding 3 - 4 foot long antennas is pretty easy. An 8 inch tall antenna is going to be way to short to work, even over 2 miles. On top of that, mounting it low behind a truck cab is going to hamper performance even more. Not only will the cab block the signal, the lack of a proper ground plane is going to severely reduce performance.
Probably the easiest way to do this is to use a magnetic mount antenna and just remove it when you need to. Expecting an 8 inch tall antenna to work effectively on CB is going to be verging on the totally impossible.
As for an NMO mount, good choice, can't get more standardized than that. You can get a Larsen NMO-27 antenna. It'll have an NMO mount, but will be close to 4 feet long. They work pretty well given a proper ground plane. I've used those on top of pickups before.
I guess another option would be to mount the full 108 inch whip on the rear bumper and bend it over and hook it to the cab. Used to be a pretty popular method back in the 1970's.