I'll second the L-theanine suggestion. I've used it with Kipp for thunderstorm anxiety and myself for school stresses and it really makes a difference.

I might just stop everything with her. Ignore her unless she does something great (went on her mat herself) and then go to her and give food. No petting or anything, just a surprise treat.

You may just be "too much" for her, too high energy and confusing -- she is afraid to fail, which just translates into plain afraid.

Betty, I agree with you on this.
I was also thinking that she could also be misinterpreting interaction (laughing, doing dishes, debating a sports team, etc) between you and your husband as disapproval, especially if she's that noise sensitive, doesn't understand what's going on and is taking it to mean she is bad so her confusion is causing her fear to increase.

She got blood drawn today and we'll have the results by Friday. The vet says she appears to be very healthy. She was very enthusiastic about greeting him.

Despite moving often over the years, her routine has stayed about the same. She's always been very active with me and the other dogs, and Clark has gone along when he has the time. Could a dog really just suddenly change to where an old routine is too much for her?

I wonder if there are noises or something that are bugging her at your new place?

How are you enforcing manners with your her/ your other dogs? Is she possibly seeing how you act with harder dogs and thinking you are scary?

What if you spent some time simply giving her HIGH VALUE rewards for no particular reason?

"could a dog really change so much that an old routine is too much for her?"

I don't think so. If you really haven't started doing anything new then I agree it's super weird. No shock collars?
Have you suddenly taken up shooting sports? One of my old dogs wouldn't come anywhere near me if I had been handling a gun. I guess dogs can smell "gunshot residue" or whatever they call that on TV crime shows.

We don't use any kind of electronic device, prong collar, choke chain, or correction collar of any sort with Shelby. We have tried them out with her in the past and she just doesn't really need them. She's soft enough that a stern no when she's very bad works just fine, although we haven't had to tell her off in a while.
No new hobbies either! There are firearms kept inside but they haven't been fired in over a year.

Clark mentioned today that River's been exceptionally butt-headed since she's in heat. It got me thinking that I've read that the chemical change in an intact bitch can start a week or two before she actually goes into heat. That would coincide with Shelby suddenly becoming super paranoid, anxious, and fearful but I've never heard of a spayed bitch reacting that way in the presence of intact ones. Does anyone here have experience with that?

The day after I wrote my first post, Shelby acted incredibly happy to see me and wanted to do things with me. The next day, she didn't want anything to do with me and was content to lay on her bedroom bed all day. I've taken the advice of leaving her alone unless she does something exceptional and I think that's just how I'll leave it for now.
Her blood work is normal and the vet suggested that the l-Theanine can't hurt to try, and if it doesn't help within a month then we could try prescriptions.
I told Clark he needs to step up exercising her himself since I won't be doing it. I pointed out the times that she has gotten into trouble was directly because she didn't get enough exercise for many days due to his schedule or laziness. I think we're on the same page, just a matter of him actually doing something about it.

BTW I wanted to point out that River is being a butt head toward ME. She is NEVER allowed to bully Shelby in any kind of way. They play well together and I have them take frequent breaks so that things don't ever escalate. River is the kind of dog that likes to push and push and push until the other dog snaps and then River pushes harder and it becomes a fight. She hasn't done that to Shelby and I insist it never happens.