I find it is cheaper to spend the money to diagnose the issue properly. Drive that Chevy 4x4 till the mercedes issue is properly diagnosed. There has to be a better benz mechanic in Des Moines right. - let him diganose then if it is a warranty related issue let the dealer do the work. Throwing parts at the problem is ridiculous - get a second opinion or make the dealer only charge you for parts and labor that fix the issue properly. Best of luck and keep us updated.