If you haven't already replaced the original 2-piece spark plugs with improved versions, do that first! Better yet, pay to have a Ford dealership or well-insured repair shop do the work. If a 2-piece plug does break, that means pulling at least one head, so the cost of having someone else do it is cheap insurance.
$1000 isn't much. After taking care of known weak points like the plugs, I'd invest in new fluids. Power flushing the coolant and engine oil is a good place to start. Take samples of the old oil, and have it tested by a lab to see what telltale stuff is in the oil. If you have a spun bearing or other nasty thing, you'll need more money, but it's better to take care of it sooner than later. If not, you have peace of mind.
Next go for the transmission fluid, filter and cooler if there is one. If the trans is slipping, now's the time to fix it. Otherwise a new filter and fresh new ATF will help keep it running strong. Don't forget the transfer case! Treat it to a complete oil change, using the latest synthetic or semi-synthetic oil.
Finally check the differentials, and treat them to fresh gear lube. By this time you should have had plenty of time to find and fix any other problems related to the 4x4 drivetrain, steering etc.
If your brake fluid looks murky, and you still have money left, flush your brake lines and put in new (DOT4 if possible) brake fluid. This probably should be first on the list...