Yes, If you put it in 4wd to stop the propshaft turning, turn one the other will go opposite way. This way you will be able to get a idea if the Sun and Star gears have been damaged. If you take it out of 4wd the Crown wheel and pinion will turn if there is a level plug in the diff case you can check the teeth as you turn it. Have you drained any of the oils and looked for debris?