How old/big is it and how long since it was planted?

the beauty of a willow is that if you take a branch of in winter and plant it about a third in the ground and water it in the spring it will become a tree so if you cant move the whole thing you can move as bigger piece as you can manage .

I'd imagine you're actually referring to the Kilmarnock Willow, which is grafted onto a standard stem, and not the true Weeping Willow, which gets absolutely huge and is often seen by lakes. I'd echo Drc's query - if its been in the ground longer than 2 or 3 years, difficult to move successfully.