The original nuts for the latch are crimped on the corners to grip the tailgate sheetmetal. But over time they come loose and fall out, usually down into the inside of the gate. Here's the original setup:
On first generation trucks, you can only work through the hole created by removing the center unlatching handle; and you can't really get nuts over to the latch bolts.
The best fix I've found is to remove the original square nut-serts, and then alter the square holes, by adding a slot, like this:
And then recontouring the metal a little bit so that you can now use clip-in nuts, like this:
It's not that hard to make this modification, and now you can replace the nut clips easily if it's ever needed. Otherwise, once the factory nuts break loose, you're screwed.
If the original broken nuts are still in the gate, you'll have to dump them out through the removed handle hole, or fish them out with a magnet pickup tool.
Hope this helps, and good luck.