Plywood might work one time in a pinch, but risky given the weight of the 5er pin added to the truck. I'm guessing the 5er ain't small if you are pulling it with a Freightliner. My concern with plywood at that weight is splintering, which could impale one of your new tires, and may damage the sidewall of one of the 5er tires if you approach the plywood at an angle.
Best solution would be to drop gravel in that area, 3/4" or larger. Kitty litter will disappear in the mud...traction is only half the problem, sinking in is the other half.