I have outlined my favorite VHF antenna for offroading before, but I will do it again as it has been while.
When I set about designing my radios for my vehicles, I went in with the intention of designing antennas that refused to break. Even if I rolled my truck, they will work. I figured that I could go with something very stiff that could shrug off the hits, or something very "floppy" that could just bend out of the way. I tried the floppy idea by stretching a CB antenna spring out. It worked great but it would lay down as I traveled down the highway and it was hard to tune up at VHF frequencies.
I eventually came up with a design that I now use on all of my heavy off road vehicles and my 4 wheelers. I have NEVER had one break and have beat the ever loving crap out of them. I have even used them as tow point to pull stuck 4 wheelers.
You will need a very strong steel plate as your mount. Very heavy duty (1/4 inch thick is about right) and secured to the vehicle very well.
Drill a 1/2 inch hole and mount one of these into ithttp://www.hamstick.com/275.html
<--- standard 3/8x24 CB type mount.
Then find some old 102 inch CB whips for cheap. Cut them down to 19.5 inches (leave it a little longer to tune once it is installed) and screw them into the mount. If you are worried about skewering yourself, drill a hole in an old golf ball and glue it onto the end of the antenna.
As you can imagine, this antenna will not break. It MAY bend, but it takes more than a few tree branches to do it. I have rolled my 4 wheeler with mine and it is just fine. If you really get to beating them up, the mounts can sometimes break but you have to sheer it off on a low hanging bridge or something. They are very broad band as they are so thick and they tune up great on 440 as well. Plus they are easy to repair on the trail. Everything can be removed externally and replaced quickly if needed (very rarely though).
And you don't have to worry about them laying over at highway speed
Give it a shot. Ugly but it works