Did you ever check the egr valve? I would check it first before I went to the hassle of dropping the tank. When the car cold and off, pop the black cover off the egr and stick your finger up under the egr. Press up on the diaphragm and relase to see if it moves freely. If it does, take the hose off and press up on the valve. Put your finger over the nipple and see if it holds vacuum. If it fails either test then replace the valve. Start the car with the hose off and press up on the diaphragm (careful, it gets hot quickly) and see if the idle changes. If it doesn't change the egr may not be seating properly or the passages may be clogged and it may just need to be removed and cleaned. If it does change then hook the hose back to the egr and see if the diaphragm moves. If it opens at idle the you probably have a bad control solenoid.
I've had the same kind of problem with my car twice. Both times it has been the egr. The valve was carboned up and wouldn't seat, which didn't set the ses light, and the second time the solenoid was bad, which did set a ses. Either way it wouldn't hurt to pull the valve and clean it and it's a lot easier than dropping the tank.
