Ttub is right, get the engine good and warm to expand the aluminum in the heads. The Linear Temperature Expansion Coefficient of aluminum 22.2 (10-6 m/m K) and steel is 13.0. That means as your cylinder head gets up to temperature, the head will expand 70% more than the spark plugs and it will loosen accordingly.
Be sure to use antisieze on the new ones. There's also the potential for bimetallic bonding, which is where two dissimilar metals will fuse. This can cause galling in the threads on the head when the offending spark plug is eventually removed. Unless your Sequoia has been experiencing heat issues or the spark plug insulation has been compromised, it's not terribly likely, but an ounce of PM is worth a whole lot of swearing down the road.