Axle won't go in far enough

I assume this is an A body rear? Axles should be the same length, 27 11/16". I suspect the second set of splines might not be lined up. Try turning the axle and apply inward pressure like pishta suggested, you may have to tap on it a little as well.

The suregrip has two sets of splines in it.

X2... Generally when an axle won't go all the way in it's cause the 2nd set of splines aren't lined up with the 1st set. Turning the axle to align them is the way to do it but just so you know it's not easy to turn. I use my 3' prybar jammed between the wheels studs to give me leverage. Screw a couple lug nuts on the studs back to back for the prybar to pry against so you don't damage the threads.