the problem with the tom-tom units is they are made for use in town not in the backcountry. there are large screen units that work offroad and some of our members are using them so maybe they'll chime in. i use an older model magellan meridian that is loaded with topo maps along with street maps. good for offroad.
problem is magellan no longer supports the meridian so other then used you won't find any. 
magellan does offer the explorist that replaced the meridian and several club members are running them so you can get some tips from them. garmin make numerous handhelds that work well for jeeping too in the etrex family. i'd say look for a unit that has topo maps and a removable memory card. the units without cards tend to have limited memory and eventually you get to a point where you have to delete old tracks to put in new ones. just my opinion on the matter. i'm sure you'll get lots of people with differing opinions. 