I've read allot about the syncro problem.
The cheap 30 dollar ones (Doorman) have too small a roll pin in them that can sheer off.
The specs on them are not very good either, they are usually quite sloppy, even from new.
Many here say don't scrimp and get the Motor Craft brand, but shop around for a good price.
Mine is in good shape yet, and I plan on squirting a bit if Tri-Flow in there at least every 6 months to keep it that way.
The sleeve bearing dries out (usually makes allot of racket) becomes sloppy and chews the sensor all to pieces as well as the tooth that passes in front of the sensor _ like yours.
They go for a long time like this before any codes are thrown up.
Technically the thing is still working, although it may not be telling the computer the precise moment to inject the fuel in, the truck will still run well enough and go unnoticed.