Do you mean the bumper stop? Rubber block about the size and shape of a soda can attached to the frame rail directly above the axle?
Not a difficult replacement if you have the ability to get up into the wheel well. The stops on my 2006 chassis are toast and replacement is on my winter maintenance list. For my chassis model and year
FORD lists them at $35 a piece but I think I'll see if I can find something made of a better material that will last a bit longer.
2006 Hurricane 31D built on a 2006 Ford F53