It is an o-ring with a funny outer sealing lip. It is fairly easy to replace. It is called a shift(er) shaft seal.
The transmission in your car is a Ford C4.
It requires the trans to be drained and pan dropped. Then the valve body has to be removed and there is a nut inside that holds the shifter shaft in place. Once you get that nut removed it can be withdrawn out the side of the case and the o-ring can be swapped out.
When you reinstall the valve body, be sure to tighten down the bolts from the middle outwards or you can get low pressure to it and bad shifts causing your frictions and steels to glaze quickly.
In my 73 station wagon the transmission mount has to be removed and the transmission tilted back somewhat, because the shifter shaft cannot be withdrawn from the case completely due to lack of space between the case and the trans tunnel.
While you are in there, I would recommend doing a filter change as well.