this is the way to start, Jesse. find out if the door is actually level.
if it was bought used, who knows what might have happened.
unfortunately, this is very hard to detect via internet. like the other poster said, you have to approach this yourself and do your best common sense judgment.
i am assuming you have more gap down at the pillar B door jam, and less at the top. Pillar A is where windshield is installed, on the side. Pillar B is next to your left shoulder. Pillar C is the rear glass one. Pillar B is shared by 2 doors, front and rear, on four door cars.
anyhow, doors, if the sag naturally, tilt downwards, with pillar B lower door edge going down. if that's the case, and you want to try brutal force approach, you can find several suitable stable pieces of wood, like 4x4s, and stack them underneath the lower door panel edge. not the door skin! the solid metal, the flare. make that stack taller than distance from the panel to the ground, with door swung wide open. jack stand the car, then, position that stack so that open door will rest on it, and start slowly lowering door onto the stack. basically, you are using car weight to force door back up. you have to go very slowly, tediously, and recheck on results many times.
rod through the hinge, even if it has small ground areas in it, may cause some sag. keep in mind, it's ANGULAR CHANGE. a little bit of sag at the hinge gives a lot of change at the door edge.
btw, latch in pillar B can also be adjusted. usually, those are held in place by 3-4 bolts, when loosened, they permit a lot of slide in latch. every car i have seen so far has this. you may try fooling with that too. as that latch pulls door with it, when shut. plastic pieces in it might have worn out.