Bad axle.
I can almost guarantee it.
Feel around the boots on either end of the axle... you will find a split in one of the boots, from which all your grease escaped and dirt/water got in.
You will need to replace.
That said, they can go a while once torn before catastrophic failure.
A bad axle can sometimes be "reset" by pulling over, reversing for 20' or so, then going on down the road. Will start clicking/thunking again in 20min or so.

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