Is it the extension housing bushing number 96?
I don't believe so, I think that reference (#96) refers to the bronze tail shaft bushing - which is why I am replacing the tail shaft housing. The tail shaft seal has been leaking and I felt the bushing was worn. I decided to upgrade to a tail shaft lube modified housing with a new seal and bushing.
When I (finally) pulled the old housing off, this unidentified part was on the tail shaft. It appears to be a silicone rubber that has swelled with age. I presume it originally fit the tail shaft splines tight, but it is a very loose fit now.
I couldn't find that part number in any manual I have and Google didn't help.
The general consensus from the tranny gurus on FaceBook is that it is the temporary factory seal/plug used to contain fluid and block dirt until the driveshaft yoke is inserted at vehicle assembly. Similar to the infamous temporary dipstick plug often found in the transmission pan?