[MHeadling]

Yes seems a popular option but depends on driving style/track, if it's all carpet/astro then go for the gear diff, if it's a mix of wood/carpet I would go for the ball diff

3k is a good starting point, to stop my one leaking I put some hermatite sealant on each side of the gasket (degrease all parts first before adding the sealant) and this seemed to do the trick