Byron,
Steam cleaning has sort of been out of favor for several years. Hot water (190 F) high pressure washers seem to be the "thing". I have used both and there are just the usual dangers such as forcing a liquid into electrical connections, into u-joints, etc. With either you want to keep your wand moving baby; otherwise, you can take paint etc. off. The idea with the steam or the hot water is that it heats say baked on grease making it easier to remove than just a cold water washer. These machines can be purchased or rented. To make heat they use #2 diesel fuel, propane, or I ran a steam machine that used natural gas. They are normally powered by either an electric or gas motor. Here is a picture of one of my machines. It is very old and I doubt the company is still in business. As you can see in the picture I had it out wheeling. Most now are much smaller and put out the same psi. There are probably companies that do this kind of work but I will say doing an underside of anything is an ugly, wet, dirty job. Hope this has been of some help.
Bill