"zerk" fitting on piggy back of shock...

gotcha

 

If your north enough to be in the snow belt, check with snowmobile shops as most of the newer machines have nitrogen shocks on them, they might have the ability to charge or check your shocks

 

If there are no mountain bike shops in your area, try somewhere with a TIG welder. You can fill the shocks with argon. Expensive but works as well as nitrogen.

150 psi is fine. Remember the gas is simply there to accommodate internal volume changes caused by shock shaft intrusion into the shock body.