You're gonna have to go to a pressure washer at a self car wash. Put on a bathing suit and goggles (I'm not joking), and lay underneath and start spraying. It will take a longgg time!
This is I used to do back in the day. Park your truck in the street, take the garden hose to it and place a sprinkler under it that will spray up. Turn the hose on and it will do the rest. You might have to move it to get all of the undercarriage. Worked for me!!
