installing a driveshaft oil seal

[David M. Hoffman]

'95 G20 stock auto

I replaced the right driveshaft about one month ago and now the
transmission-to-axle oil seal is leaking (a few drops a day).  I checked
the seal before I reinserted the axle but I apparently missed some
damage.  My Haynes Primera manual and a quick inspection has convinced
me that it will be nearly impossible to insert a new seal properly
(manual says 5.9-6.1 mm positioning from the trans case).

Questions:
Has anyone attempted to replace the seal with the trans in the car?  How
difficult was it?  Because of the cross member and exhaust positions, I
don't see how I can drive the seal into place with a socket let alone
get it positioned within 0.2 mm.

Is there anything in the FSM that might help me?

Is there an after market seal that has a built in "shoulder" to help
position the seal in the case properly (like the seals on drum brake
wheel bearings)?

Thanks.