You shouldn't be able to slide the yoke/ball housing at all if it's attached and sealed properly, it should be snug and allowed only slight up and down movement when the torque tube/rear end moves.
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0451.htm
Here's the seal replcement procedure:
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0712.htm

If Gus (DADS50) chimes in his seal replacement is very well documented.

Allan
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1951 GMC 1/2-ton
Two 1953 Chevy 3100 5-window