Ford Expedition engine miss
I have a 2000 Ford Expedition 4X4, 4.6 L, 60,000 miles, that is missing when I exelerate. It does not do it all the time, maybe about once every 5-10 minutes of driving. I realize the chore it will be to simply change plugs on the truck. I understand it may take up to 2-4 hours to change them. My question is, should I spend the money to pull the engine codes to see which cylinder is missing, or should I just replace all the spark plugs and see if my problem disappears? Today, I added some Sea Foam to the engine to see if that helps. Will know in a few days, I guess.
Also, the truck growls sometimes when going up hills. I had the truck in for service and the mechanic told me the differential gears are controlled by a computer and that they needed to be told to mesh tighter together. They charged me $500 and sent me on my way. It still makes the noise and I am wondering, have I been taken advantage of? Does the computer really tell the differential gears how much pressure should be on them? Any other ideas as to what the noise may be? It feels as though it is coming from under the floor boards of the passenger side, but I am not exactly certain. Please advise?