pooky

hi im new to jeeps i have a 97 4.0 and once upto temp it creates a miss fire seems to be overfueling as if you try and power through the miss fire it back fires and just gets worse if you let off the throttle it gets better and sorts its self out for a while,my jeep also runs on lpg and has this problem all the time whilst running lpg ive replaced the plugs and rotor arm any sujestions to what is going on,also whilst towing my boat i get a grinding noise from the rear end???
this is my first jeep ive always had range rovers in the past and as much as these probs with the jeep are anoying at least it starts lol