AJo can be reasonably folded UTG. Once the OP limped, the problem is he got great pot-odds to call a smallish raise (esp. since he could bank on the MP1 to call as well).

While this flop is scary, I don't think that a made flush should worry you so much with the A and K both on board. What two diamonds would be making or calling raises preflop? Q10? 910? Probably not.

The more likely scenario is that someone has the nut-flush draw with QQ or AQo.

Either way, as much as you can't really ask for a better flop for your hand, the likelihood that you're beat or that your J-high flush won't be good is too high in a 4-way pot.

I either lead out at that flop to show strength and hope that people are playing weird hands or I play weak-tight and check-fold.

The way you played this hand left you with a difficult turn having a strong hand with redraws to a full house but up against likely holdings of a straight/set/nutflush draw etc.