All my 4wds stay in some sort of dirty fashion. My Tacoma is going at least 5+ years without a wash. I did have to clean the rear glass off yesterday though because I couldn't see anything out of my rearview mirror.
I will say the Bronco was the dirtiest new vehicle I ever purchased. The amount of dirt on the inside was ridiculous. Like they left the windows down on dirt mountain for a month or two.