Clean it and clean it and clean it then dont be surprised when 2 years later you still find mud...their is no way to get rid of it all. But yes, pressure washer and i have heard simple green works as well.
I think that these steps you follow for removing mud
Apply a small amount of mild liquid detergent to the stain.
Blot the stain with a dry,white cloth
Flood the area with white vinegar
blot again with the cloth Apply a little more liquid detergent to the stain
Blot the stain again
Blot one last time
dont pressure wash under the hood everywhere else yeah sure but be very careful under hood throw liberal amounts of water on a fairly cool engine bay scrub with your choice of soap rinse repaet as neccesary
if you bag the distributor you can pressure wash everything. but for future refference, you can get a custom distributor bag from almost any ford taurus in a wrecker. do not prolong washing it. make sure you blast out behind the tires in where the important stuff is...like the brakes. if the mud has dried, soak with a grarden hose before dropping a 1000lb load in the local car wash. make a list of them and check off as used. you will be banned fom use every time...lol you will need to spend atleast 15 bucks to do it right so you don't cook things.... and if your gonna be playing around in the mud... loose the front backer plates or you will get rocks stuck in your rotors and kill your pads. don't forget the rear breaks.
your distributor should be sealed well enough to keep the water out while pressure washing...my advice, go to local car wash, leave your truck running and pressure wash under the hood and then engine, if you have an open air filter(like a cone type) i would advise against doingthis but if everything is stock under the hood go for it, when water hits the engine it basically steam cleans your entire engine, i've done this on every car i've ever owned to clean the bay, NEVER had a single problem, but you HAVE TO LEAVE YOUR ENGINE running, if the engine is off and you spray it with water, the sharp temperature change could crack your block or your manifold(s)
Some utility style vehicles have rubber mat throughout, so its easy to open the doors and hose the mud out, And others may pull out all the seats, tape and seal any electrical connections in the wiring harness.