Sometimes the dip tubes can be deceiving. If they've been locked down for a long time, they look almost like they're welded, and part of the keg. They can be hard to remove.
Make sure to get yourself a wrench that fits the posts and knock them loose. This some times takes the combination of a socket wrench and a 3-pound sledge.
Remove the posts and then reach into the keg and push up on the dip tubes to break them free. THen you'll see the small gaskets.