I am assuming that the plug broke off level with the cylinder head, leaving no metal part with flats to be able to use a socket to remove it; otherwise you would probably not be in need of help...
It's kind of a weird thing to happen, actually, I dont think Ive ever had to deal with a plug broken off so low down... Usually the threads strip out of the head before the plug metal body ever breaks. If the plug was siezed in that tight, then the head may need to come off for repair anyway(not a terribly involved job, but you dont want to do it if you dont have to). Lets see what you can try here.
If there is a portion of the round plug body remaining above the hole yet, there are specialty sockets ("turbo sockets" Snap-on calls them) to bite into and remove rounded off fasterers and studs, etc... These sockets look like they have a spiral shaped hole in them, unlike conventional sockets, and work very well at most things. Don't use a cheap one (chinese tools); it will probably just round things off and not work well.
If teh plug is broken off flush with the head, you really have no choice but to try to drill it. On this side, removing the exhaust manifold may be required first to gain clear access to the hole. This is one of the steps for removing the head anyway... <smile>
Take a set of good quality hardeded steel (cobalt or similiar) drills and starting with something small, start drilling in the center and work the hole larger by using consecutively larger bits. You DO NOT want to get any metal shavings in the cylinder, if you do you will probably need to remove the head anyway to get them out or they will cause major damage to the cylinder walls on startup. Spreading a coating of grease around the hole helps to catch the chips and keep them in place. When the hole gets big enough, a large easy-out can be used to remove the plug.
If all of teh above fails, remove the head and take it to a machine shop; they can fix it easily once it is on the bench.
A word of caution: Dont break off an easy out in the plug, it will make it very difficult for the machinist to remove. Also, be absolutely sure not to drill into the head when enlarging the hole, this will create more damage.
If this is an old beater truck, and it's the 8 plug setup, you may want to just try driving it and see how well it works with only 7 plugs. the worst damage I can think of happening might be a coil failure, or a remote possibility of taking an ignition module out from an overheated old coil as a result, and it probably wont pass any emissions test. This isnt a very professional or craftsmanship way to do things, but if you are stuck and cant fix it properly for some reasdon at present, it might be something to try.