Early Sure-Grip Questions

You might not get a lot of help on this here because many don't realize that
the early units had split splines. I am not sure increasing the axle will do it
unless you are using green axle bearings because the center thrust button
controls the engagement.

I had the same set up you have but went proactive. You can buy a replacement kit that swapes out to the full engagement spider gears for
about 180, which in my opinion is the way to go. If you do that, use the old
cross bars as I have heard stories about the new ones not fitting perfect.

PM if you want more info.