yea,the martack is a spot weld on the splines to keep it from going too far into your diff and causing damage to your diff and from pulling too far from the birf..It elminates the need for the PITA snap ring and make birf swaps easy..Marlin used to have it up on his site..Here is some info i have on it..
Its an old article and as far as I know marlin does not do this for free anymore..
http://www.off-road.com/toyota/cheaptricks/clipweld/
