Are you sure that axle is installed all the way in? Can we see it from the side view? Some back and forth is normal on an older high mileage rear. Stick with what you have and repair it if something is broken. Someone offered you a core if I remember, get it and rebuild it your self with the help of utube. Or buy one rebuilt and ready to install from TJ. The 9 inch rebuild videos are basically the same for your 8 inch unit. Are you able to pull the center section third member if it is in fact broken internally?