This is my girlfriends truck, so whatever I don't know about it now I can find out.
I believe it's a 3.0, automatic.
This Ranger has sat for about a year now. I jump started it and it runs but won't get above 2000-2500 rpm in drive, and when it's maxed out it peaks and drops over and over again. I checked the voltage of the battery whIle the truck was on just to make sure the alternator was feeding it properly.
After I shut it off, and tried to start it again, all I get is a clicking sound. I tested the battery with a voltmeter and it read above 12v before I tried to start it again.
I have limited mechanical knowledge, but I learn well. I'll find an answer to any question. Please help me troubleshoot this Ranger.

 

a fully charged 12 volt battery (right off the charger) should read about 13.2 volts. It'll read about 12.7 after sitting a bit. If you are getting a much lower reading than that, your battery may be bad. Jump start the truck and measure the voltage across the battery terminals. If your voltage meter reads somewhere about 13.8-14.2, your alternator is charging. If the battery is dead when you jump start it, your volt meter could read much higher than that (above 15v or more) while running, until the battery is sufficiently charged.

Clicking usually means low voltage or a bad connection. You should carefully check all connections and your battery cables for green fuzzy corrosion, and make sure all your connections are clean, bright, and tight.

Once you get it starting reliably, start diagnosing your run problems. Is the CEL on?

 

+1 ^^^

If it sat for a year without starting and battery was connected then battery is most likely bad.
Car batteries can't be drain all the way down and then come back to life.
They are made for high amp discharge(starter motor) then quick high amp recharge.

Slow drain batteries are called Deep Cycle batteries, not great for starting engines but good for long slow drains in trailers, and 2nd batteries in boats and RVs.

Yes, test battery after it sits overnight, 12.3v is lowest it should be, 12.2v or under is time for new battery.