I would guess gearbox teeth stripped as well. Slight possibility locking nut on the tapered splined output per shaft could be loose and let the blade carrier/stump jumper drop down enough cause slippage? Ken Sweet
I would recommend tightening as you did and then retighten all 4 after using a couple hours. Also, you may want to double nut each axle if you have trouble with them backing off and getting loose. Ken Sweet
Deere dealers probably leased out a good many of that size as it is very popular in HP for so many applications. Then several buyers could not make the payments and let them go back to hit the secondary market? Ken Sweet
Theoretically, the pins in the lower holes will let 3 point on the tractor lift the cutter higher than in the upper two. I have never seen a setup like you describe? Ken Sweet
If you are mowing a unknow field, you may want to double the hours mentioned above in order to have time to react to possible hidden objects. Ken Sweet