Demand drive in the front diff. Rebuilding the front diff is actually very easy. I did one on a 10 500HO and it was two hours from start to fully reinstalled. There really is not much in the front diff so the hardest part is getting it out of the quad. You have to remove the wheels, undo the tie rods from the strut, disconnect the ball joints from the A-arm and then remove the axles from the front diff. Axle removal can be the hardest thing. Prying from behind while pulling helps get them off. I made a puller with a muffler clamp and use a slide hammer to assist with removal. mount clamp on big round part and pull like hell. LOL
Once the axles are off you need to drive out the roll pin on the prop shaft. With both axles and the prop shaft out, then there are 4 bolts underneath that hold the diff to the frame. Take it off and take to the bench. you remove the bolts and pull it apart and you will see the Sprague cage and the rollers, springs and armature plate. Make sure plate is flat. If the cage is cracked it will allow locking.
All that being said, I am pretty sure that changing your fluid will likely solve your issue.