You can replace the extension housing seal, but it will leak again within 2000k's, take the extension housing off and replace the bush and the seal at the same time, the bush flogs out and then the seal goes, so just replacing the seal is just a band aid fix, the tailshaft flaps around in the extension housing flogging the seal out again.
Just make sure you align the hole so it lubricates, there is a special tool for fitting these, you might have to get a transmission shop to fit the bush, just take them the housing and the bush.
https://www.ebay.com.au/itm/REAR-EXT...gAAOSw8btgR6Xe