Ticking under load

My '89 MJ 4.0 has this awful loud ticking noise under load. For instance, if I rev it in N or Park, it's as quiet as a mouse (aside from the FM40 but I digress), or cruising along on level ground. As soon as I hit an incline or accelerate though, it's back. I've tried different weights of oil, and have searched, but it looks like flexplate issues are mostly just starting out until it takes up the slack. I don't believe it to be an exhaust leak or anything of the like because I checked everything out and would also expect it to tick in park or neutral. The other thing that bothers me is this vehicle only has 50k miles on it so I sure hope it's nothing internal.
Any thoughts?

It's not uncommon for the exhaust manifold to crack at the Y where the two secondary pipes meet. You may wanna check that out. The noise that comes from a cracked manifold is exactly as you've described and can mimic pinging.

You might've already checked, but, if your throttlebody or air intake manifold is heavily clogged w/dirt or carbon deposits or oil, it will ping & cause loss of performance on acceleration. Later it'll get worse & you'll have a rough idle, but at first it's only noticeable on acceleration under load. Hope this helps.