I go mudding a LOT, and the best method for cleaning my under carriage is just using a strong hose nozzle to get off the majority of it, then just using a scrubber for what didnt come off with just the nozzle. If you want to make it spotless, hand wash it with window cleaner and a rag.

 

just put it on a lift and power wash

 

tire foam works good if you want to give it a shiny affect