how is it sticking, are the pins not retracting all the way or is there something out of line with the latches themselves?
my tailgate sticks and it its due to a bad handle or rather the mechanism behind it not moving the rods the proper distance... it has a sweet spot on it that i can find and open it but nobody else has any luck
so it could be that or i would look at the latches and strikers on the top rather than down at the hindge both truck side and gate side to make sure they are not corroded, worn, or bent. also make sure the plastic piece isn't missing from the striker post (post part on the truck). also look at the tailgate and make sure it isn't warped. check all the bolts and make sure they are tight. lastly have ever greased it? sticker posts and latches on the doors and tailgates are moving parts just like everything else and they need lube... but nobody ever lubes them.
if all that fails then your probably doing to have to figure out what is misalighned and shim it