I have a 2007 EXP EL 5.4 4x4 and at 45mph with the cruise set, when you slowly accelerate (most noteable with the cruise button) it bucks and makes a clunk like the transfer case or the drive shaft is getting hammered. I had Ford test the coil packs after reading on here that they can misfire when they get weak. They found that 1 and 7 were dropping out so I replaced them with FOMOCO coils and the problem was still there just not a severe.
So I went back to the garage and they tested it while driving and found 3 was dropping out. I replaced #3 and the problem is still there and I would say just a bit worse than before. I am tempted to just replace the rest of the COP's but not sure as this seems weird that after you replace one a new one drops out that did not show up on the test just 10 minutes ago.
You should consider doing the plus too. Similar story here - 2 weeks after purchasing my 07 4x4, same thing. I replaced all plugs and coils and it runs smooth as can be now. I did use motorcraft plugs but chose to use Accel COPs.