Hi Steve.... If you said the transfercase front driveshaft/output shaft seal they should know what you're talking about. I had one leak a while ago and replaced the seal but it still wept. So then had to get a Speedy sleeve fitted as the shaft was a little grooved but it wasn't that obvious. Just saying to have a good look at it while it's out.
I didn't have a speedy sleeve so replaced the flange bit that the universal bolts to which fits on the splined output shaft. I think that came from Don Kyatt also. The centre nut is a bit special. 36mm castellated and flanged (on FG649). Broke a breaker bar trying to undo it. Neighbour's rattle gun worked. The nuts/bolts for the flange cost arm and leg, I used Nylok - deeper than original but there was enough room.
Cause of my failure was possibly the very worn splines in the drive shaft. I could feel the vibration. If not the cause it contributed. Had it rebuilt (and lengthened a bit to compensate for spring height) by Hardy-Spicer in Darra.