Those MIP style shafts you listed require a different end for the cup on the diff, fyi. That bone is only half the part.
I don't know what new savage's come with. I bought mine used and it had the 8 shock setup and small outer axles/cups with machined steel small tooth diff gears.
I've upgraded mine quite a bit.
Sintered BP diffs
Super HD outer bones/diff cups/wheel axle cups
LRP30
LST HT Pipe with HPI's curved header (vs the one with a nasty kink right off the engine.
4 plastic big bores with hellfire white springs and I think 50 weight oil
T-bonz rear skid w/ wheelie bar
OFNA slider linkage mod
Hitech 985MG t/b servo, some high torque digital JR steering servo
Dual pink fiber brakes
RPM center skid
OFNA 125cc tank to do away with the HTL issue
I ran an old HPI/HB rear center CVD in it for years, but after putting in the 30, I kept snapping the pin in it, so went with a dogbone instead.