You shouldn't have to pull the heads to get the lifters out. However, you WILL have to loosen or remove the rocker arm support shaft assemblies. If you do this, make sure that you keep track of what push-rods go where, and which head which rocker assembly goes on (I'm pretty sure they're interchangeable). You want to make sure you put everything back where it came from.
Do you notice any "play" in any of the valvetrain? You'll probably have to rotate the crankshaft by hand through a couple of revolutions, and check for loose parts every 15-20 degrees of rotation. If you don't find anything this way, then pull off the rocker arm assemblies and check the rocker arms, valve stem ends, both ends of the pushrods, etc., for visible wear-and-tear.
The lifters in my '94 are roller lifters. I assume the '91 has got 'em too. Check the roller on the bottom to make sure it's OK, then have a close look at the pushrod "cup". If the lifter is not "pumping up" properly (which would cause valvetrain slack and noise), then this cup may be depressed into the lifter. You might try pushing down on each pushrod (into the lifters) before you remove everything, to see if one of them is noticibly easier to depress. I think I've read on some other threads that Ford has had some lifter problems with some model years, but I don't recall any of the details. Might be worth a search, though...
The full top-end gasket kit is expensive!! ($120-170 - yikes!) Intake and valve cover gaskets will hopefully be cheaper. Having just had the heads off of my motor, I can honestly say that you don't really want to go there just for the fun of it...