Burnouts are not the way to test it.
Get the front end up on some jackstands. You'll be able to tell what's happening with the CAD. You might have a broken shift fork if it's still the OEM aluminum.
I usually test mine by driving it on pavement. Put it in 4WD, turn the steering wheel, and drive a short distance. You'll know immediately if it is locked in. Not everyone would recommend that, and you can damage things. As soon as you feel the first kickback from the U-joints, pull it out of 4WD. Test over. Nothing damaged.