[drubie]

They are quite easy to change if you have access to them. First measure it and buy the right size (most auto parts places have them in the common sizes). Get a couple just in case you mess it up.

Next step: remove the old one. If your exhaust is swiss cheese then get it off and out of the way to improve access.

Once you can reach the freeze plug, you need to jimmy it out with something. They are press-fit into the hole so you can normally stick a screwdriver through the middle of the plug (if it's rusted out like they normally are) and just lever it out. Then smear a little Permatex Formagasket on the new one and carefully hit it into place. Generally I would just grab a block of wood and hold the plug in place and hit the wood with a hammer although you can buy a specialty tool if you're feeling saucy. Don't distort the plug (this is why you might want to buy a few if you've never done it before).

It's not a technically difficult job if you can reach the plug. If you can't (like the ones on the back of the head) you are in trouble and the motor/gearbox generally has to come out for access.

edit: My SJ used to only run hot when it was basically out of coolant. I would check/replace the thermostat, thermostat housing (they corrode), the water pump might be leaking and/or ineffective (the impellers corrode). Flush your radiator out while you have the cooling system open, some of the SJ's had a pretty hard life and half of your radiator will likely be blocked with various "mechanic in a can" solutions. A leaking exhaust would not cause overheating.