From 30 years living on the beach: Cars and boats, keep them washed and waxed ( painted and chrome parts ) rinse as often as practical with fresh water between washes, including under the vehicle. I would not spray wd40 randomly under the vehicle. It will protect but will also catch dust / dirt which will hold moisture and make it harder to clean next time. Spray as often as practical with fresh water.
If there is a particular area where rust is starting you could use wd40 or even better get some crc spray from a marine supply place. There are several kinds. One I use is similar to wd40 in thickness, the other is almost like spray grease. I spray the ob motor bracket with that where it will be sitting in salt water. CRC is more expensive than wd40, but much better corrosion resistance.
Basically if I decide to spray something I spray cheaper items with wd40, expensive items with CRC. Newer cars / trucks are much better about not rusting than older ones, unless they come in direct contact with salt water which a real no no. Yours is a 1994 so you may have some places you need to spray if they are already starting to rust.
