Sounds like the timing belt is going bad and the motor is out of time slightly. this motor is a dual overhead cam design and it is pretty tricky to change this unless you are a good mechanic.
have they check the timing on it. sometimes when timing is off by a few degrees it would cause both rough idle and backfiring. also check for a dead cylinder.
when my dodge stratus started to do that it was because I needed transmission fluid.
It would be very rare for a P1684 code to turn the light on.
It might be that your pcv valve is plugged or the egr valve is sticking.
Could be a number of things CONSULT with with your Dealer and have it looked at.
No.
Failed fan motor, relay, wiring, fuse, computer.
Loose belt - will sound like a screeching noise.
If if starts bubbling even if the engine is cold, most likely blown head gasket. If only when engine warm, stuck thermostat.
Well a lot of them have the same engines & transmissions; but if you have a lot of people you'll be driving around, you'd want the grand caravan...The only difference is the length. I would go with the standard size, it'll probably get better gas mileage and cause less wear on the transmission & engine.
Low trans fluid, or internal trans failure.
Noises are impossible to correctly diagnoise unless you hear them. TAke it to a good diagnoistition.