When you get your computer code from Autozone WRITE IT DOWN
you need to know the code, not what the counter guy said.
The chacoal canister is likely mounted right near the gas tank, sometimes above the spare tire. I am not familiar with the 04 Mounty so thats why I dont know exactly.
emissions control tank, is not a term that I have heard before
the charcoal canister however stores gas fumes from the gas tank until they can be purged into the intake manifold and burned (for emissions reasons)
When you get your computer scanned write down the OBD II code and google it