If AJ had mentioned the truck to crime stoppers when he first saw it on Wednesday, LE could have not only zeroed in on the suspects sooner, but they would have been able to make their arrests before more evidence could be hidden, like the gun MS buried. Then maybe we would have known whose fingerprints were on the gun.
Personally, right up until Tim's ashes were found, I had hopes that he was still alive somewhere, did AJ not have the same hope? I would have hoped that if he did, he would have said something as soon as he suspected it may have been Tim's truck, thinking he might have been able to help him. To me this is similar to the 'Why didn't DM tell LE as soon as he could if he was innocent' argument. As it has been pointed out here, you only have to fear someone while they are free, once they are in jail they are no longer a threat to you, theoretically.
I almost find it suspicious that he knew for days that DM could be a suspect in a major crime, that he took photos of the evidence, verified it anonymously, and then did nothing about until after he was fired and it was too late. Those photos would have been perfect for blackmailing, actually. Not that I'm saying that is what he was planning, just that it is a possibility, logically.