I just got a 94 xj with 210k on it. It runs fine but heats up to redline in a matter of five minutes in cold weather. I understand these things are infamous for running a bit hot with the 4.0 crammed in there. I replaced my Thermostat with a new one and a fresh gasket, after finding it leaked, I resealed it with a better gasket and gasket sealer, it leaks a little bit but its a loose hose on it and its barely. I find it still has the overheat issue, so my question is what should my next step be and how can I diagnose if its something more serious like a blown head gasket?

 

Red-lining is not "running a little hot", it's a major malfunction.

If you have a loose hose, you should tighten it up. That won't solve your problem, but seems foolish to NOT correct something that simple.

You can check for oil in your anti-freeze or vice versa - if it's visibly obvious that there's cross contamination then you have a bad head gasket. There are test kits you can buy to check for a blown head gasket as well. You can also use a compression test tool to check each cylinder for compression (it may be available as a loaner tool)
That said, the I6 engine is freakin' solid - it's definitely NOT known for blowing head gaskets left and right like some lesser engines. Certainly if you're running it red-line hot for some time or very frequently, they're not indestructible. 210K is nothing for these engines - my Dad's 94 XJ ran to 400K original miles and the engine was still running perfectly when he junked it.

I'd immediately check your fan clutch and aux fan operation. I read somewhere that when the engine's warm if you can spin the mechanical fan, it's bad. The aux fan should kick on when the engine hits about 220. Also are you getting flow from the water pump?

I had a 2000 TJ and an aftermarket water pump I put in had a plastic impeller. While driving a long road trip the plastic impeller broke. The engine would obviously heat up to the red line after a few minutes of operation. Other than that there were no typical "bad" water pump signs, the seal wasn't leaking, the bearing was smooth, but there was obviously zero flow. If you pull the radiator cap and run the engine, you should be able to see the coolant moving. If it's not, then you need to check the water pump.

 

Use a radiator pressure tester and see if the system holds pressure. Basic tool for testing cooling system. If not, you have a leak, hopefully not into a cylinder via a bad head or head gasket. But at least you will know. Then if you are talented enough you can also pump up the tester and look around for the leak too.
Also sounds like you may need a new thermostat housing. Dorman sells them about $15. They are soft aluminum. Once they go they're just gonna leak.

 

Regardless, I'd suggest fixing the leaking hose, if that comes off/splits, you will have a big mess on your hands very quickly.
It's generally considered routine to replace the water pump at the same time you do the t-stat. There is a built in indicator of a failing water pump. It's called the weep hole. Even if the bearing hasn't completely poo'd itself yet, it will be on it's way out if you start getting coolant coming from the weep hole on the front of the water pump housing. If it's really bad you can see it while the engine is running. Or if you catch it in time, you'll get a few drips every time you stop somewhere.
When you replaced the t-stat, did you make sure to orient the new t-stat with the air bubble escape hole to the top? If it's at the bottom, like the f***ing nitwit previous owner on mine, that'll help your jeep heat up pretty quickly as air won't be able to escape out of the system, will get stuck in the block. And we all know air pockets can cause heating issues, it's just physics.

As mentioned, if there's an option that has a plastic version of any kind... you DON'T want it. All metal impeller version will set you back a whopping 30-40 bucks tops for a good one. And a couple hours to install. If you need help with that... just PM me. I just finished both of those on my 99 XJ.

 

Yeah.. always good to replace those it's cheap, even if yours looks like it's okay.... the smallest corrosion on the mating surface and new gasket or not, it will most likely leak.
There is one brand, wish I could think of it right now, but ask them... it's one that if it fails, it fails in whatever position it's currently in. (Fail safe I think is the bad one) Have heard of some of them failing closed. That would definitely give you fits with overheating.
When checking for the water pump... mainly look at the underside of where the lower radiator hose connects to the pump, as that is where it will run down and onto the ground from.
To make sure it's the pump and not the lower hose, use a flashlight and look for a coolant trail up above where the hose connects. That's where the weep hole is. Even if there's no noise, coolant only starts coming out passed that point when the bearing is starting to go. Kind of like the replacement layer in a brake pad that intentionally will squeal to let you know it's time to check/replace before damage begins to occur.

 

Definitely heed the advice of RocketMouse and 97Grand.

It really comes down to:
The water pump moves the coolant. If it's not functioning properly, you'll overheat.
The thermostat allows coolant to flow through the whole system, opening as the engine comes up to proper operating temp (210). If it's stuck closed, you'll overheat.
The fan clutch (primary) and electric fan (auxiliary) force air across the radiator to remove heat. If they're not functioning properly, you'll overheat.
If you have a lot of air in the system, you'll overheat.

In order to red line the temperature gauge, you have to have a problem with one of these things in a pretty big way.
A little leaking coolant, a bad radiator cap that's not holding pressure are problems, but they wouldn't cause a quick red-line.

Again, double check that the gauge reading is accurate using an infrared thermometer as suggested. You don't want to just throw parts at a problem that's not even there.

Even a leaky head gasket probably wouldn't cause a quick red-line as described, but I'm not an expert on that sort of thing. My Dad's XJ ran for 400k original miles without an issue, my TJ ran for 305k original miles and both engines ran perfectly when they were decommissioned.