I wouldn't mount anything heavier than a bumper sticker to the tailgate. It's not the weight alone; it's the metal fatigue where a weight is attached. Eventually the sheet metal will crack around the bolt holes.
If you decide to go ahead, you should:
1) Spread the weight around - build your mount so that it fastenes to opposite sides of the tailgate, and at the top and bottom.
2) Make your bolt holes go all the way through the tailgate.
3) Make the inside hole about 3/4" in diameter. Then weld a bushing to the inside.
4) Weld stiffeners over the bolt holes, inside and outside - use a patch of sheet metal, about the same thickness as the tub, about 2" square with rounded corners. Seam-weld it all the way around.
If you do all that it will last a long time before breaking out - probably as long as you own the Jeep.
And here's an alternative for your jack. It keeps the jack out of the trail slop and nasty weather, and away from larcenous eyes, but it still only takes a few seconds to stow and retrieve.