Part of my old job for a fire suppression company was checking extinguishers.
How to test if a dry chemical extinguisher is still good:
Check that the gauge is in the green (duh.) Not over or under.
Flip the extinguisher over and palm the tank. After a short period of time the powder inside...
It will work if there are no fender flares, I mounted to the metal body of the truck. They will likely have to stick out more and will look more like the Subaru guys run them. Also I woldn't recommend them if you have larger than 32" tires.