I could not open the links so shooting from the hip here. You mentioned a few things, Wheel off the ground, tires rotate opposite, Diff is not locked. Tires on the ground, drivetrain loaded, Diff is locked. You heard a click. I hope I am wrong but you may have something broke in the differential not allowing the spider gears to rotate freely under load. If theres anyway possible to get a telescoping magnet down into the housing and look for metal particles, you may be doing more damage by driving it?