First, I cut out the ruined sheet metal where the hinge broke off. That left about a 2-1/2x3" opening.
Then I could see that there's enough space
behind that panel for a new, stronger mounting plate.
So if I needed to mount the hinge like so (the yellow), I figured I'd need a plate shaped something like the blue outline. So I drilled (4) 3/8" mounting holes (red).
Then I cut my new mounting plate. I used about 8" of 2x1/4" flat stock. If you were careful and spaced your holes closer, you could get away with less, maybe 6", but I tend towards overkill.
I had to notch the bottom corner to clear a bulge in the unibody, then I drilled and tapped four holes to match the ones I drilled in the jeep body.
I ground the old welds off of the hinge, drilled a couple more 3/8" holes to bolt it to the plate, then (after test fitting everything) I drilled and tapped the plate to match.
Then I shot it all black.
It took a little wiggling to get it into position, but the plate bolted in nice and solid.
Then I bolted the hinge to it:
And finally rehung the door:
