Can you tell where the movement is located? The axle should absolutely shift back and forth, there is a term called "fully compressed length" which is important when comparing axles. If you wanted to compare the length of two axles you might say, "they are the same length fully compressed" - this is because it is designed to have movement back and forth and just sitting them on the ground for comparison can be misleading.
As Andystoy19 mentioned, you need a real solid push to get the C-clip to lock inside the housing. If the inner CV is moving a lot, it might not be locked in place. If so, you should be able to remove the axle easily. In my case though, once it was locked in, I did still have some movement back and forth and I could see the inner CV shift back and forth away from the diff. It is not much though, less than a 1/4" for sure. This is different than movement within the joints.
One last thing, when you lift the truck, or do suspension work, it can stretch the axles out. One way to know if you have too much is to check to see if the axle will wiggle back and forth. As long as there is a little play in the shaft you should be good. Just adding this as supporting information that being able to move the shaft back and forth is normal and ideal.
