Power to each rear wheel is provided through a CV axle from the rear differential. CV axles typically don't get sloppy like a "U" joint. But I cannot say "never". I advise to place the rear frame of the car on jack stands (not the rear suspension), disengage your parking brake, and turn the tires gently "back and forth" to feel the slop of rear wheels. Watch the CV connection at the differential while to-and-fro to check for "play" within each CV axle. If each CV axle looks good but there is a difference in the amount of "play" between rear tires, you might have a problem with your differential, emphasis on "might". Other forum members may provide the diagnosis if there is a difference in the amount of "play".

Then repeat the process, but this time look at the input to the differential from the drive shaft. If it does not move, then the play is within your differential. If the drive shaft is synchronized with the motion of each wheel, then your "play" is on the forward end of the drive shaft. You would have to remove the tunnel cover to inspect the forward area.

I did a quick search and found pictures of Sky drive shafts with and without "U" joint connections so that is confusing. I hope others will clarify if there are "U" joints on each end of the drive shaft. Logic says there would be, but maybe it is a different type of joint.

The hard part of the back half inspection, is getting your rear up on jack stands, which is of coarse not hard.

 

This just sounds like the infamous "transmission clunk". Probably 50% of our cars have this and it was much discussed back in the day. The majority theory is that it is just slightly undersize shims in the diff. GM has said it's nothing to worry about and very few people (not racers) have had diff failures.

If your car clunks when starting (or slowing) in first gear, or in 1st - 2nd shifts, that's what it is.

And in answer to your first question, our cars do not have U-joints. They do however have CV joins at both ends on the rear axles.

 

You may have a loose retaining bolt on the drive axle or the suspension. The hub bolts also have a history of loosening, and that could manifest as a clunk, as could a loose bolt or failed flex coupling in the propeller shaft.

In my Sky experience, CV joint failure starts as a clicking sound, and not a clunk.

Redlines have the transmission-to-axle strut, and I have not heard of them "clunking" in the classic kappa sense.

 

sounds like nothing to worry about unless it gets louder. Right now, if I am paying attention, I hear it occasionally, not all the time.