If you jack up the truck (front), one side at a time and attempt to rotate the wheel with 1st gear and diff lock engaged there should be resistance - if there isnt you've found the side with the problem. If theres no resistance either side, its the diff. If its not the whole diff then:
Getting the inner driveshaft out of the front axle requires you to remove the wheel bearing, stub axle and outer drive shaft. If the outer shaft and CV/swivel are ok, try to remove the inner shaft - you may get just a piece if its broken. From then on your talking taking the diff out to remove the snapped piece.
Let us know what you find and good luck!