If there is plenty of liner left on the shoes, they're probably glazed. This can happen when brakes are lightly applied (and if you operate your 770 like I operate my 790, you do use the brakes lightly rather then throttle down sometimes).
So, you need to break the glaze (or live with the chatter). One way to do this is hold the brakes (fairly firmly) down while driving. This is easier if you can do it on a straight road. Or, you can scuff up the liners with some sand paper (wear a respirator if you do this) to break the glaze.
I've never had to do this with my 670 (770's little brother) or 790 (770's big brother) yet, but I have done it with cars and trucks I've owned over the years.
I'd never experienced this problem when cars had brake shoes but I have seen it occasionally with vehicles with disk brakes. Doesn't seem to happen too much anymore due to changes in the brake pad materials.