Make sure to check the bulb itself anyway
I have the same issue (rear left turn signal is out, and the front left one blinks very fast). I noticed that the rear left brake light would come on though. So I removed the tail lamp housing and looked at the physical bulb itself. The bulbs used in the 2001 expeditions has two element wires inside, one for the brake light and the other for the turn signal.
So you could test the brakes and determine that it's not the bulb, when in fact the turn signal element wire in that same bulb could be burnt out. The owner's manual says the bulb to replace it with is a 3157K. Looks to be about $6 everywhere I look online, so not an expensive fix by any means.