As far as the plastic guard sliding, it shouldn't do it too easily by hand. If it's really easy to move, requiring almost no effort, then it's not really working as intended. It's there to keep larger objects from finding their way into the axle tube, but doesn't in any way keep water, sand, or fine mud out. This is why it's highly recommended to drain and fill your diff if you do a lot of water crossing up to that point and deeper.
I've got a set of alloy outer axle seals up front. It's an extra nipple to grease, but does a better job of keeping water out.
Alloy USA Grande Axle Tube Seals, Red*ALL-11105 by the Jeep Specialists | Morris 4x4 Center
(wrong link for 27 spline axle shafts, but you get the idea)