Dammit, I hate being right like this. Now I feel bad about the tools, b/c they aren't going to be much help to you for a good long time.
Unfortunately, the top of your engine looks almost exactly like mine did. While I didn't try to actively move my jeep while the rod bearing was failing, I did try to get it above idle (blip the throttle in gear) and it didn't want to at all. On mine, the #3 was the wet cylinder... #4 was the rod that jammed a hole in the side where the EGR had recently been mounted. But I don't know if that really mattered at all in the scheme of things. I pulled the whole HEAD before it imploded, and couldn't find diddly, so I doubt you will find much of anything either.
Got a hoist or a strong rafter above that engine bay? Time to start thinking of ways to support that 500 pound brick and lever it out of there. I'd say you are about to be arse-deep in this thing, pulling the engine and having a shuffle at the bottom end. The only advantages to doing it now - NOTHING HAS PUNCHED THE BLOCK YET. This is a big one however, because you can rebuild what hasn't been broken, and from what I understand, VM Specialist may have just about everything you might need. Repairing the bottom end also might be a lot easier because of the "tunnel" design of this block - Once the oil pan is off, it *should* be capable of being worked on without too much more disassembly. Whether you can get the pan off without removing the whole engine from the body... Is a good question that I can't answer. The tunnel design is supposed to allow the crank to be replaced while in the vehicle, but maybe nobody told Chrysler that little tidbit.
As far as a compression test, I doubt that will tell you much of anything. If you fancy a go at it anyway, DO NOT USE THE STARTER (too dangerous!) just use shop air and a rubber-tipped blowgun. Put it into the injector hole and hold it down, and push the button. Fair warning: You WILL need to rotate each cylinder to 90 degrees ATDC and use a socket wrench on the front to hold it against the frame - The crank WILL ROTATE with the pressure of the 100 psi shop air.
I actually built a compression test rig using a pressure gauge and valve with the rubber tip - That way I could do a quick "leak down" test on each cylinder. FYI: None of the valves showed any significant flaws, and the $60 I paid to a head shop confirmed that. They did a vacuum test on each valve, said that their gauge read 'good' from 5-6... Each of my valves pegged the needle at 8. Combustion backfire loss into the intake also wouldn't be a ping or tap... It would be a deafening bang with fireball. I doubt you have that either. Don't forget, I thought mine sounded like a misfire and possible injector failing. The injectors also all checked out within spec.
Here is my engine, with its own suicide imminent:
https://dl.dropbox.com/u/5447172/jeep-engine.movSound anything like yours?