Well today is the first cold day I drove my truck since I bought it. I noticed it takes a while to warm up. It'll get a little above the C line within 5 minutes of running, but then won't go much higher than that, even when driving. The heat is luke warm, and I noticed, when I parked it at the gas station and kept it running, the heat got a bit hotter. I'm pretty sure the thermostat is stuck open. Would anything else cause this? It's getting a bit chilly here in MI, need to get this fixed.

If it is the thermostat, is it pretty easy? I've only done thermostat work on chevy's. The motor is a 4.9 I6.

Thanks