What year and what engine?
If it is an '87 thru '90 4.0L. that isn't an overflow tank, it's a pressure tank. If fluid is overflowing, the first question is whether or not it's filled to the correct level. It should be filled to the halfway mark when cold. Look inside the cap and you'll see a post under the fill opening. There's a notch cut out of the post about halfway up -- that's the fill level.
If it is boiling and overflowing when filled to the correct level, the cap isn't holding pressure, or the bottle itself is cracked. Although you can buy just the cap, several people have reported that putting a new cap on an old bottle doesn't work. Quadratec sells the bottle and cap as an assemble for about $19. That's probably what you need.
Just remember that the bottle is part of the pressurized cooling system, it is NOT an overflow bottle. NEVER remove that cap when the system is hot. (If we're talking about an '87-'90 4.0L)
