Side note....I loooove your Ranger, very well done.Typically those shocks should be good for about 100k normal driving miles.
If racing on them, it's a good idea to rebuild with new seals every race.
it's really easy to rebuild these shocks. I've rebuilt my RaceRunner coil-overs about 5 times now, the hardest part is resetting the piston in the reservoir. Although I have heard that Fox's are a little pain in the ass, during disassembly. Snap ring inside, kinda tricky to remove.
You should never have to recharge them, unless you blow a seal in the reservoir, or you take them apart to rebuild them. they should have a normal shraeder valve, like a rims air valve. but they are filled with nitrogen, not air