Is it just "harder" or does it stop and not go in all the way?
Do you have a limited-slip diff, or locker?
Sometimes the limited-slips and some lockers have two side gears on one side that need to be re-aligned to get the axle splines all the way in. Very common issue in the back for the 9", but not sure I've ever heard of it being an issue in front diffs.
Good luck.
Paul