It is probably a Dana 80 (4" axle tubes) and to tell what gears he has there should be a metal tag under one of the cover bolts that will give the gear ratio (4. 10 or 3. 54). If there is no tag then a person can pull the cover and see what is stamped on the ring gear or just jack it up and count how many drive line revolutions to one tire rotation. Hope this helps.
Mark