After 206k miles, and some scalloped tire wear, I figure its time to replace the rear struts. The accordion-shaped "dust boot (?)" is also disintegrating. Two questions:
1. Is the scalloped wear due to worn struts or rear end misalignment?
2.I can replace the struts myself. After replacement, will I need to get a four-wheel alignment?
Thank you.

 

Both rear tires scalloped?

Dust boot break down is normal.

Do a strut test by pushing done on the car then releasing (like pushing on a swing) to get it to oscillate as much as you can then release. Determine how many up/down cycles the body does before stopping. The oscillation should stop pretty quick and not go on.

Check strut for fluid leakage. Some minor signs of oil at the strut rod are normal.

Check the suspension for worn bushings, etc. Sometimes things can get pretty loose.

Are the tires balanced?

Proper tire inflation?

Any inside or outside abnormal tread wear?

Cheap tires?

Do the strut test first. The internet has videos on this. Then inspect struts and suspension.

 

No excessive bounce. I do see minor signs of oil on the shaft, especially without the dust boot intact. Tires could use re-balancing, not done since installation. Proper tire inflation. Abnormal wear: left rear tire is "scalloped" on the left edge, right rear tire is worn on the left edge. Not cheap tires, Michelins.
I watched a You Tube video where the guy described it perfectly as the Wump Wump noise, and mine is even MORE noticeable at slow speeds like 10-20 MPH. He swears the scalloped wear is caused by nothing else but worn shocks/struts.
Another symptom I forgot to mention is that when going over bumps, there is somewhat of a "clunk" noise from the rear, as if a loose object bounced in the trunk.
I figure the tire with the scalloped wear will still produce the Wump Wump noise until replaced, even if the cause is fixed?

 

Clunk could be worn sway bar bushings a common issue. Typically, one cannot tell if they are worn by looking and thus just replace them.

Still clunking could be a worn suspension arm bushing (does happen) and here you should be able to tell by using a pry bar to check for play.

Inner or outer tire wear can be too much camber.or toe angle. Look at the car and see if the tires seem to be angled with the ground. One some cars this stands out as camber is used to get a better road grip but will cause tire wear issues.

Check the suspension for issues and make repairs.

If one tire is scalloped, I would give that suspension a close inspection and wiggle the tire and other suspension parts for play. Suspension bushings can also wear out. Sometimes a wheel bearing can fail to the point of excess play.

If both tires had inner or out wear and the tread was still good (inner/outer wear now causing the tire to need replacing) I would get an alignment AFTER doing any repair work.

Struts are cheap and easy DIY. Might shop online for lower prices. But look at the suspension first.