Two possibilities, is it the leather or is there a small hole in the sole? I had a pair of Meindle sheep boots that squeaked, sounded like leather on leather but ended up being an airhole in the sole. Found it by immersing the bottom in soapy water, then flexed the boot and looked for bubbles. Found it easily and a dab of silicone took care of it.
My 13" Schnees squeak like crazy from use. When leather gets worked against another pice of leather it takes on a shine and is like rubbing wet fingers on glass. Like bicster said, easiest thing is pitch blend or petroleum jelly, etc. Taking some sandpaper to the smooth leather works too, but not for long.
The annoyances we have to deal with, eh?