Front diff problems

Hello everyone
I have a 08 grizzly 700, yesterday i tried diff lock but it would not lock for me. Today i went out to look at the front end and the axle where it goes into the diff moves side to side when you would think it would be more ridged. Anyone know what could be happening? How hard is it to pull the front diff? Would it be better to order a good used one or rebuild mine?
Thank you for your time and any help you offer
Matt