Ofna spider availability seems a bit low but it has the strongest internal gears.
I'm using a Slipperential atm which I don't recommended for a Savage as the slipper part isn't really up to it.
Looking to try a Vorza\Hot Bodies CD which i bought fairly cheap on ebay. There are a good range of spurs available for it, I have a Hellfire 52 tooth lightweight spur on it.
I saw that you posted about an Ofna center slipper. I'm really starting to think that i don't need\want a slipper at all in the center and they always seem to be troublesome. I have the slipper locked up on the Slipperential atm and using Ofna silicon diff lock fluid in it. This provides a near locked up CD driving experience but when landing on one end on power allows some power to the redistributed to the other end of the truck instead of instantly shocking the drive train. No drive train breakages for me yet.