The main trouble is that you want the expensive hard parts covered but don't want to spend much money.
In any case, the simplest solution is to just talk with your agent and ask him straight out what they will cover. From my experience your own insurance company is pretty reasonable to work with if something happens to the truck. Back in the late-90's my K5 was stolen and had quite a bit of aftermarket stuff (though it actually looked decent at the time) including tires and wheels, lift, aftermarket bumpers, lights, etc.... I had receipts for all of the aftermarket stuff and also for any recent mechanical work (new batteries, etc....). With proper documention on my part and being reasonable they took care of all of the aftermarket add-ons when the vehicle was recovered (it had been beaten, off-roaded, and stripped).
Another factor regarding insurance I commonly see is simply whether you want to keep it legal for street use or not. Here in Ohio you have to at least have liability to get it registered. I know several people who scoffed at this idea and say "screw it, it's just an off-road rig" and don't bother getting plates for it. That's fine for some people, but depending on where you live it can greatly reduce your options for fourwheeling. Our club has about 4 decent trail areas all within 1.5-2.5 hours away, basically a reasonable distance to just head out Saturday morning to wheel all day and get back home at a reasonable time. However none of these areas have any decent trailer parking at the trailhead and require 5-15 miles of pavement between parking area and trail. Plus all of these legal areas are unmaintained township roads so technically you could get ticketed for an unlicensed motor vehicle running them. Take away the license on my K5 and now I'm left with only the choice of one place that is 4 hours away and anything else is at least 6-8 hours away, and that means substantially less wheeling trips each year.