Some rotational play in the yoke before resistance is normal with a lock right... how much play that you now have I can't say. There shouldn't be any lateral movement of the pinion yoke. The pinion bearings should be holding the pinion shaft tightly.
Has your pinion seal been leaking? With a lot of lateral movement in the pinion shaft I would think it would be. Might be a silly question, but are you sure your pinion nut hasn't backed off (loosened) on the pinion gear shaft?
Depending on the mileage of the diff you have, the bearings might just be getting long in the tooth and worn out OR possibly you might not have gotten your side gear tolerances on the bearings set tight enough which caused more bearing wear.