I believe the locker actuator on the C303 axles are a vacuum diaphragm. So vacuum strokes the diaphragm against the force of a spring, then opens to atmosphere to allow the diaphragm to be pushed back into place.
For anyone less familar, the solenoid valve allows a vacuum source to go through the valve to the line to the actuator (when energized), and then when de-energized switches the line to the actuator to connect with the third port on the solenoid, which is just a vent at atmospheric (0 psi) pressure.
When it does this, the vacuum in the lines wants to pull air back in to allow the locker diaphragm to sit in its normal position, wherein lies the possibility to suck debris in at the solenoid, The three way solenoid will have three ports, (one from vacuum source, one to the actuator, and the third should have a drain line or vent filter to keep crap out)
The thing is, it would only want to do this when you de-energize the solenoid, which usually isn't when it's sitting in mud or water, but if your vehicle stalled or things were caked with muck there is some risk. I would just mount the solenoids somewhere dry in the engine compartment, maybe up on the firewall or something and leave 4 inches of tube hanging vertical off the solenoid vent or get something like this
Aliexpress.com : Buy 10pcs 1/4"NPT Male Pneumatic Muffler Filter Sintered Bronze Silencer Vent Brass from Reliable pneumatic muffler suppliers on FY Eletrical&Tool Store
The vaccum actuator valves you mention may be for emmisions and are just a valve allowing vapor to be pulled into the coal cannister or intake to burn them, but may work if used together. The only thing is you would need to find one NO and one NC when energised, so that you wouldn't need to power one or the other all the time, or just get a 3 way.
