Altitude
 

My truck runs terribly at altitude. It sputters for a bit, catches and runs fine, then sputters some more. It seems to happen around 9500ft. When I put it in 4x4 it runs better but not great. Any ideas???

higher octane fuel and maybe higher weight oil?

When at high altitude , disconnect the battery when the engine is cold , for about 10-20 minutes . Reconnect it and start and let it idle till warm. Drive normally for a day or so to let the PCM relearn it's optimal settings.
If that doesn't help , post back here .

You can actually use lower octane fuel at high altitude. I doubt that will solve the problem though.

Rangers will automatically adjust the fuel ratio at higher altitude with its sensors. I don't think altitude is sensed but air mass in the intake and O2 sensors on the exhaust keep the air/fuel ratio near optimum. Maybe your MAF sensor is out of range at 9500 ft.