FROM PAST EXPERIENCE!
Your PTO drive shaft attaches to a gearbox on the top of the tiller.
From that gearbox, trace the drive train/shafts all the way to the tiller shaft.
CHECK EACH AND EVERY GEARBOX for gear lube.
Buddy of mine has a John Deere 1035E with a tiller to match. (factory kit, trailer, tractor, tiller!)
I was sitting in the tractor seat tilling a food plot with it when it "shelled"!
It was about (+/-) 10 years old.
ALL gearboxes were bone dry!
It was so bad, it couldn't even be rebuilt!
Had to junk it and purchase a new tiller....all because me, him or his wife didn't know to check the lube level in each gearbox. "I" should have know better, but I thought the gearboxes were "sealed" against lube loss. Instead, they were only sealed against weather intrusion.
Had we checked the lube level, we could have just had it rebuilt. Probably twelve to fifteen hundred.
Instead, had to spend $3600 on a new one!
A piece of farm machinery is relatively tough, though it can be abysed. With routine maintenance, a piece of equipment should last a long time.
Longer than 7 seasons, anyway.
P.S. - the new one, a King Kutter is checked for lube levels frequently!