I've been researching this as well. From what I gather so far complete rebuild kits may be difficult to find. Here's an interesting read I found which gathers parts from different vendors to make your own rebuild kit http://gbodyforum.com/viewtopic.php?f=19&t=35708#.UNoRhrbTPaY
Most would simply stick an Eaton or Auburn unit in but I would also like to hear if the resident experts have a source for complete rebuild kits.
the clutch pack was around 100 bucks but is discontinued for years now
and without the clutch pack you cant rebuild it
switch to the eaton with multi spring spring pak and rounded ear discs and if you wear it out the clutch kits for those are still available around 135.00
Yeah the link is old but according to the Nitro Gear & Axle website the clutch packs are available, though, I have not contacted them to confirm. If not, then I'll just switch to the Eaton unit you mentioned.