Check:
-Truck end of the pigtail- make sure the female end has round holes. If they are not perfectly round, it wil reak havoc on the speedo.
-Pin going from the adaptor into the transmission adaptor is square. They get rounded with age, and again cause problems.
-drive gear in transmission, and driven gear on the part that goes into the transmission that the adaptor box or VSS hooks to.
-my post last week on this very subject; I went into greater detail. I am kind of winding down and not real talkative just now.
Daniel