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Turns out converting a TF's altmode from "off-road" to "monster truck" is just as much of a chore as I expected it to be. Limbs, joints, clearances... Nothing ever fits right. Forget raising the suspension, I'm knocking my head into the wall just trying to figure out wheels!

I don't feel skilled enough to re-shell a figure, so that's leaving me with wheel/tire replacement, and I'm hitting a wall. Cybertron Dirt Boss was an idea, but I don't think I'd call it a good idea. Energon Tow-Line is another possibility, but while easy, also feels like a cheat.

Does anybody have any suggestions that would make a good monster truck without compromising the transformation?

Not sure if it's what your looking for, but the Power Core Combiners line had Mudslinger, a scout class monster truck:

They also made a blue version named Salvage.
On other forums, people have had luck attaching large LEGO tires to Siege Crosshairs, and one person convincingly placed a wheel-less RiD Strongarm and T30 Roadbuster on a toy monster truck's base.
Hope this helps!