Here is a easy way to fix your shaft wobble, even if only temporarily. The male end of the middle shaft just needs a single layer of electrical tape.
The tolerances on the Minelab shaft are actually pretty tight. If it was any closer you might not be able to get it apart again. Anybody who has swung a Tesoro Knows those twist-locks only work for first assembly, lol
I've done this before with other machines except I used aluminum tape, time will tell how long the electrical tape will hold out. The tape in the picture has about 15 hours on it, so far it's OK. Besides, it's easy to replace.
Coincidentally when I took the machine apart for a cleaning and to take this picture, I noticed that Minelab made a similar raised surface on the lower fiberglass rod. The lower rod is so long that a little play really wouldn't translate into as much slop as the upper connection does.