The front tube of this lens is only held on by three screws with plastic bushings that
ride in machined slots in the zoom body. Excessive axial play is a sign that the screws
are becoming loose or the bushings broken or worn.
To correct this the lens must be disassembled.
Remove the rear assemblies, front element group and
outer zoom grip tube as shown in the master gallery.

The screws in question are actually "upside down" and must be
loosened/tightened from inside the lens.
The middle element group mounting ring blocks the holes provided
for access in the zoom body and must be removed.