When I had to replace the flywheel ring gear and clutch on my '46 2 ton, there was so much mud, gunk, and actual animal pucky under the truck, the transmission actually appeared to ba about twice the size it really was.
It was in January, and it was mightly cold out, and I did all the work right out in the open in the wind.. so pressure washing wasn't much of an option.
I used a few different sized putty knives, some wire brushes, and at least in one spot, a chisel and hammer.. and a whole bunch of WD40.
I bought WD40 in the gallon can, and got a squirter on a hose that you could screw right into the gallon can.. that worked out best, since not being aresol based, you could spray upsidedown, and realy get all around it.
It took a couple hours, but by the time I was done, the tranny was clean as a whistle.. and I didn't have to worry about the stuff freezing while it was loosening up the crud.. and it really penetrated thru the grease and oil soaked, hardened muck.
Not sure i'd use that method for the entire undercarage unless I had a whole lot of time on my hands, and a whole lot of WD40.
In non-freezing conditions, I really like using Engine Brite, then pressure washing it off.. but effective and efficient "no water" solutions are limited.. especially when your trying to clean off decades of road gunk, mud, oil, etc.
