742 8-3/4 Sure Grip Rear

Ive been having some problems with my duster and what I thought was clutch slippage turns out its in my rear, Im being told my axles are to short and are not reaching the second set of splines in the rear, Im told there are two sets of splines in each side. Is anyone out there familiar with this and how do I ensure the axles are into the second set of splines
Thanks in advance