I work for a fishing company, and we have a 1972 Chevy pickup with a 454. The rear drive shaft is turning, but the rear wheels are not getting full power if any. the front tire was spinning and buried it self, with out breaking the back ones loose on the sand. Then once the truck was axle deep and no longer moving, neither were the rear wheels.
My guess is this is a rear differential problem? Thought i would ask before ripping it apart, and risking getting dirt in there. I don't think we would have sheared the splines, seeing as how it is almost exclusivity driven on the beach.
Answer: If the driveshaft is turning but the wheels are not, or have little power, then yes something broke in the rear differential. Going to need to take the cover of, turn the yoke and see what broke.