I use this
under-car sprayer attachment for my pressure washer usually; not after every trip, but after every few to get the chunks off...
For the wheel wells/mud flaps/skids/tires/etc. I'll pretreat with P&S Mudbuster (4:1 dilution), let it dwell for about a minute, and then rinse with the pressure washer (25° nozzle at about a 2-4" distance). For the wheels/tires however, I will agitate it with a wheel/tire brush. If the truck is very dirty, I'll pretreat everything from the windows down with Mudbuster and then rinse before foaming the vehicle
The alternative is to just use a traditional
spinning lawn sprinkler and every few minutes move its position under the truck