The most common cause of this is the 4x4 vacuum solenoid goes bad. If these hubs do not have vacuum they try to lock in. If you have a leak or poor vacuum they will engage partially which causes the grind noise. Start by checking for a vacuum leak in the system if none replace that solenoid.
I hope my assistance helped you and resolved your concern, if you need further assistance please ask.
Please note, your Deposit doesn't constitute a payment. Please click the 'ACCEPT' button to allow me to receive credit for my work. Positive feedback and Bonuses are appreciated. Thank you, Shawn.