Why would somebody have J9? There are opportunities to reraise on the flop, and it would've happened with top two pair. With that much money going in that early in the hand, stacks are committing before the turn. Maybe the 2nd caller is on the draw, but he doesn't have J9 because a raise would've happened on the flop. There is no reason to slow roll top two pair with the action in the hand. Probably an overpair (or a set) and a draw for your opponents, not top two pair.

That being said, I call this in a heartbeat. Sometimes you have to pay off, but there are a ton of hands you beat and to fold to a pot-sized all-in with a set and no possible flushes and only one funky straight possibility seems way too nitty. I think the nut straight or a set is going to milk it a bit, not shove. More likely AJ trying to protect after two weak checks on the turn and he needed a big bet to do so.

Also, you only entered the pot to try to hit a set and stack somebody. You hit your set and are worried that it's no good now? Maybe it isn't, but you can't fold it on that board. You only have to be right 1/3 of the time to break even with this call and you do have a set. There are many hands you beat and only a few that beat you.