GL1500, while changing the rear tire. What else should I check?

If you pull off the side panel on the right side, just forward of the battery, you should be able to see the rubber sleeve that hides the u joint.
my guess is there are some posts on how that thing comes out.
Good Luck

Check the right shock boot for tears. Swing arm for cracks. Wheel bearings for slop or roughness. Brake pads for wear, and slider pins lubed, especially the lower one (caliper mount bolt) behind the rubber boot.

The shock boot is now in three sections. I saw a post before on something to replace it with but it escapes me now.:sadguy:

Shock Boot

think this is it (was a long time ago)

http://www.davidsilverspares.co.uk/parts/by-part-number/part_52611MN5003/

52611-MN5-003 BOOT, RR.

If you've already got the bags out of the way, could also have a go at the desicant in the air compressor.