The best place to start would be with a steam cleaning of the undercarriage or pressure washing with hot water.  Short of those two options, we would recommend spraying the undercarriage with Berryman B-33 Engine Degreaser (part #1133) a time or two to get as much of the chunky stuff off as possible.  After that, you can remove the oil-based contaminants using a solvent-based product like Berryman B-12 Chemtool Carburetor, Choke & Throttle Body Cleaner (part #0117) or Non-Chlorinated Brake Parts Cleaner (part #2420).  Both products can damage paint, so if overspray is likely, then Berryman Non-Chlorinated Brake Parts Cleaner (part #2420-DP) is an effective paint-safe cleaner/degreaser.