What about a simple bumper guard like these?
They could go on the front or the back.
http://www2.northerntool.com/product/21061/
Molded Rubber Bumper Provide Protection
Help prevent damage to loading ramps, lift gates and vehicles. 6in.L x 3in.W x 3 1/2in.D rectangular bumper.
In Stock
Ship Wt. 6.0 lbs
Item# 151381
Discount Price... $10.99