I too do a fair bit of off-roading (http://www.landroveradventure.com
) and have been looking for something sturdy enough to take the punnishment.
I am using a Yaesu ATAS 120 on a Diamond K400 on the hatch of my Land Rover Discovery. As you might have guessed, the ATAS is way too fragile for off-road, so I take it off. No HF on the trail.
The only solution I have seen that seems to make sense is some of the stuff from Australia. They have an HF network called RFDS especially for 4x4 users in the outback. (General info here: http://www.exploroz.com/Feature/HFRadio/Default.asp
An example of what's available can be seen at: http://www.web-counting.com/cgi-bin/outbacker/scan/fi=products/st=db/co=yes/sf=category/se=Outbacker%20HF%20Mobile/sf=prod_group/se=ANTENNA/va=banner_image=/va=banner_text=.html
Hope this helps.