I have a 91 Dakota with the 3.9 and auto. I have about 125k on it and it runs pretty good for about a month after i change the plugs but then it starts to hesitate and misfire again pull out the plugs and they are burnt. I run regular 87 octane fuel, I change the oil 4-5k miles I do mostly highway driving. The truck does smoke a bit and leaks a lil oil too. If I am sitting idling for a few seconds and hit the gas i get a cloud out the tailpipe....now is this blowby oil getting in the cylinder that is causeing the plugs to foul? help they are easy to change but it's just annoying.... thanks. Also if you are local to wisconsin central area, I am looking for a fender and front clip for mine, just hit a deer the other day. Passenger side, truck is black.

 

Maybe the valve stem seals are bad...over time they get hard and let oil blow by the vavle and into the combustion port.