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.
I used to use what was called a "Steam Jenny" on a '40 I once owned. It did a great job of removing any oil/grease on the truck. But you do need to be careful about hitting anything with high-temperature steam or water. You don't want to hit anything that will melt (if HT pressureized steam which can go well over 212F, depending upon pressure), and you don't want to hit things like the distributor and wires which will cause you lots of hassles when you try to restart.
For the bottom of your truck, I would think the biggest problem would be getting away from the muck that would land on you when you get low enough to see what you're cleaning...but water would probably do just as good a job as steam in that circumstance at much less risk to anything.