If the fuses, bulbs, and all that are fine, then that leaves two obvious parts that could be the problem. Either the multifunction switch itself has crapped out, or the turn signal relay itself is bad.
Since multifunction switches are fairly expensive, toss a couple bucks at the relay instead and see if the problem goes away. Bonus points if you have another Ford around to swap turn signal relays between. If the problem moves to the donor vehicle then you know it's the flasher relay.
The flasher relay is on it's own harness under the dash. Mine is yellow, square shaped, on the driver's side. It should be hanging around somewhere near the obdii diagnostic port. Mine took quite a bit of pulling to get it free, but yours may be easier. You also may have a few clips to deal with holding the relay into the socket.