Pegged heat gauge - faulty gauge or thermostat?

[D-Day]

So, I left my truck sitting for a couple weeks being only driven by my girlfriend's dad to work once, and since getting back I took it for a drive tonight, and on the way back I noticed the heat gauge near and in the red. It would randomly drop below the red just by a hair on occasion. It has never, ever done this that I can remember. At most it would go halfway right before the thermostat "popped" and let the coolant flow through and the temp would drop back to normal.

After letting it sit a few minutes, I released some pressure with the radiator cap lever, and started it again. It was instantly back down to halfway, though I could see it creeping slightly again. It hadn't cooled down enough to check the coolant level without it bursting open, so I'm letting it thoroughly cool down. This is right before I'm supposed to leave for home, a 376 mile trip.

One thing I noticed is when I shut it off the second time testing it, the gauge would peg all the way to the right, and I mean all the way (well past red) - like when you have a twitchy ground and the gauges don't work. I can't tell if it's telling me the right thing. I could hear a lot of noises when I shut it off in the hoses/radiator from heat/boiling/pressure that I don't remember normally hearing, so who knows.

My heater was also putting out lukewarm air and not doing a very good job, lesser than normal.

The first obvious things I'm going to check are the coolant level, I'm hoping maybe it's just something dumb like that. I would have checked that before posting this, but I don't have luck that good and figured it'd be helpful to have this thread already started. If it's not that, well...I think my only other immediate and cheap option is to check/replace the thermostat.

One last note...I heard a squeak from something at two different times, like a belt slipping. That's not very common with my truck either...sometimes when it's cool and damp you get it at idle before the belts are warmed up, but never under load. Perhaps a slipping belt or bad fan clutch too...?

I don't know. It's around 15-20 degrees here in the hills of Kentucky, and even though the hills provide a good engine workout, it wasn't pushed that hard since it was snowing and I went slow most of the time. I wouldn't think a mere fan would cause that drastic of an overheat unless something else was wonky too.

Forgot to add it's a 400 engine. Anyone have experiences with this? Please tell me it's something simple and not a $300 water pump or ungodly head gasket or whatever else is out of my budget right now...

Much appreciated guys.