The sting head is slipping down and can be slid out of the shaft entirely. There appears to be a bushing missing, prehaps worn off, as this unit is 13 years old.
Am I right and if so, what part needs to be ordered and where to order it.

Hi,
The outer driveshaft tube is a unitized piece and would have to be replaced as a whole piece. Bad news is there are three, 530071425(oldest), 530069883(45") and 530069880(51")
Good luck.
Peace,
Clarke