My '91 126 has just a bit of play in the right rear bearing.
I can click it back and forth by grabbing the tire and pulling/pushing. If it was a front wheel, I would guess it needed tightening maybe 1/8 turn. I cannot feel any roughness in the bearing, but I do have a thumping sound when I drive on a rough road; don't know it it's the bearing or not, but it comes from that area.

Replacing a bearing looks to be a real job, I wonder if I could pull the half-shaft back, and tighten the slot nut a little bit to remove the slack? As I said, it's just a bit loose, but it's definitely noticeable. The left side has no discernable play. I thought I might just be able to push the axle out of the way, uncrimp the shield-holder and center-punch the nut a tap or two to tighten it a bit. Possible? Advisable?

I found it while looking for a noise from the right rear that sounds like a tailpipe thumping when you drive on a rough street.
But the exhaust is on the other side, and tight.
i'm also gonna try a new differential mount and sway-bar links.
I did the front subframe mounts last year, so they are not the guilty parties.

I read posts from 3 different pro mechanics here who have all said that 126 rear wheel bearing work without the mega $$ specialty tools is all but impossible.

The reply in this thread sorta matches up with what you are wondering about. You get a sense he's been there:

126 Rear Wheel Bearing