Going out on a limb here but on the Liberty's and on the Liberty forums, the 3.7 V6 has a problem of breaking valve springs, weak valve springs, etc. on valve springs. One of your Intake valve springs could be broken or worn so bad that the Intake valve isn't closing properly and could be throwing the miss fire code.
Of course that's a big if. How are your coils on each cylinder? Could one of some of them be bad causing the miss fire?