I definitely check-call this river. I put him on AJT with the diamond flush, the question is - is it the Ace diamond flush or the jack diamond flush. If he is sound on the latter streets as you say, he wouldn't have called the $18 turn bet and then raised all in on the river with anything less, but if you check-call here, hopefully if he has the ace diamond flush he will only place a "milking" bet down.