axle rocking in and out making noise

I have a 1998 F150 Rear axle has been making a knocking sound for a couple months. I pulled the rear tires, and break assembly. When I grab on to the rear axle by the lug nuts and do a Push/pull motion, it seams to have a bit of play, and makes that knocking sound. What should I do?? 2 years ago I changed the rear end seals and bearings, when the back of truck rock back and forth it make this cluck cluck cluck noise,please what can i do to stop it? thank you