A question for you please...

[HumminNBoatin']

Take your skid plate off clean it with a scotch bright pad really well. Use a sanding block over it again after it dries. Then spray it with the bedliner. Then bolt it back on the truck.

Use undbody coating on the rear diff. It will stick better than any paint as it is a major job to prep a rear axle correctly without taking it off the truck. Underbody coating sticks to unprepped surfaces pretty well and is very easy to touch up.