Shawn:
Most 091 trannies I have taken apart have some wear on the input shaft.
I feel the shaft is at least part of the problem. I usually have extra
used good shafts laying around, so I just swap them out, but I wonder if
you could get one of those sleeves that presses over and provides a new
surface for the seal to ride.
Installation is a pretty easy job, getting the tranny out is the hardest
 part of it. I sometimes just reuse the old bell housing gasket if I
don't have a new one. Once you've got the bell housing off. the seal
will just knock out (and the new one, in) with a driver . Use an
appropriately sized socket.
The input shaft comes out pretty easily, too, but maybe you should look
at a book to get the procedure. I once changed the one in my 82 diesel.
With its 5.86 ring and pinion, unscrewing the input SEEMED impossible,
but I eventually got it done. (You have to spin the reverse gear with
the input attached past the ring gear.)
Good luck!
Al Brase
69 double cab
87 vanagon gl

Shawn Wright wrote:

><snip>
>

>I also studied the Bentley trying to figure out how likely an oil leak
>from the trans into the bell housing is. Looks like the input shaft is the only
>place it can leak. So does this happen often? Is it an easy fix?
>I'm now wondering if my rear main seal was fine all along...
>