1998 Dodge Van backfire

After the proper fuel/air mixture is drawn into each cylinder, the piston compresses this mixture, and if everything is working properly, the spark plug fires an arc across its gap. An arc that has very high voltage, this voltage is referred to as the secondary ignition voltage. It is produced by the coil, and comes from the center terminal of it's respective coil. A six-cylinder on some, uses 3 coils called a coil pack. Your 6 is referred to as a 3.7L engine. If for any reason the spark which ignites the fuel is either timed wrong(primary controls from computer ground drivers) or the valves that are closed(supposed to be) are leaking this power, (that is actually an expanding flame or burn pushing the cylinder down), you'll have a couple problems. The exhaust valve, not seating can cause this power to leak past its valve seat(backfire), or the spark may occur at the wrong time(ignition timing) and still(backfire). The timing of the intake and exhaust valves may be off, as with higher mileage the chain(in your case) loosens. Again backfire.
So now if you understand this, you can tell exactly how the timing of all 6 cylinders are operating, and the power they are producing by feeling the exhaust with the palm of your hand. Don't try it.
In your case I think all of the things I just talked about could be adding up to cause one or more cylinders to be leaking back through your exhaust. An exhaust valve may not be seating as your giving the engine a power command. A cylinder could be misfiring from any spark occurring at any time on a compression stroke if the exhaust valve does not seat.
Why? You'll have to start by testing your engines compression, testing the computers stored fault codes. Testing the secondary ignition wires, plugs, and coil primary circuits. Even an exhaust that has too much backed up pressure can cause a backfire. A mixture thats too lean can cause a backfire, but do not be too worried. Backfires are usually found inside the distributor. You may need a full complete tune up, or you may have cylinder head vale seating problems. After all, your engine has high mileage. The number one problem I have seen causing this is cheap or poorly insulated wires, cap, or rotor problems. But a compression test and engine scanner test can possibly eliminate things you do not need to spend your money on.
If you are a do-it-yourself type person, you've come to the right website. If you need further help, take a look around at the different free information, there's soo much its quite unbelievable. To me anyway. And I've been in the business of repairing trucks like yours for a few years. Shhhhh. OK? Have a good day and thank 2carpros and Mike Cerjak for everything. I'll be around. My cars break also. Bye