2002 grand cherokee laredo backfires thru the intake. It's got a 4.0 in it.

 

Sounds like a valve problem, one of the intake valves are open during combustion, could be a timing issue or a burnt valve I'd say.

 

It could also be a shorted coil rail. Try swapping it for a known good one or even a junkyard one. Crossfiring with this system is common so much that mechanics swap them out at the slightest problem.

There are 3 coils in that rail that each fire 2 cylinders every revolution. One on compression and one on exhaust. They are molded in epoxy with long conductors. Coil #1 fires cylinder #1 and #6 at the same time and is usually the first to short out against another conductor. Depending on the mileage, it's good insurance to change it anyhow. Try it before you go firing the parts cannon.

Just my opinion and I'm sticking to it. LOL!