I can GUARANTEE you as a dog walker and pet sitter that this behavior ONLY escalates never disappears without effort.
The dog is unbalanced. Take her for LONG WALKS and enrichment. (health and weather permitting and building up to her abilities)....It builds the bond of YOU BEING THE LEADER who she can trust. That's only the beginning. IN THE ZONE WALKING, not meandering, sniffing, targeting or being paranoid of other beings or cars yada yada. WALKING. At a good pace.
NO TENSION ON THE LEASH. LOOSE LEASH. KEEP HER BY YOUR KNEE.
Get her brain OFF OF her "stuff" being what she's the most attached to.
Time for a family meeting. ALL OF YOU have to work together to rehab her so she can live an anxiety free life and not make up issues to worry about. IF THEY DON'T DO IT, then your work will regress every time she goes home.
You are correct. And it's all about calm assertive energy and working with her without emotion.
I'm not going to suggest a bunch of techniques it's too tricky. But I WOULD FOR SURE put a harness or collar and leash on her and let her watch me move her stuff and return it. Also subliminal wearing the submissive gear. (but a harness gives you the ability to grab her AND not yank her by the neck if something "happens" - since you probably aren't experienced enough for the collar.)
There are desensitization techniques. BUT THERE HAS TO BE A BOND AND TRUST.
I might also bring her in from a walk and the two of you MOVE THE BED and TOYS to all different areas. Two of you meaning YOU and her on the leash LOL. Then she sees she gets her stuff back.
DO NOT TALK. TALKING SOUNDS LIKE ANXIETY AND EXCITEMENT.
Calmly and non nonchalantly just do it with confidence. With her WATCHING. If she's EXCITED or ANXIOUS she has to sit there till she calms down. ALL FOUR FEET ON THE FLOOR. None of this front paw up in the air ready to go!
Your GOAL is to turn off the BRAIN, not simply make her stop what she's doing for a MOMENT. You must FOLLOW THROUGH until you see her give up the anxiety.
This is the same advise for all nervous and excitable dogs not just the resource guarders who can be tricky. Especially when nobody can see the big picture.
HUMANS OWN EVERYTHING. The burden is too great for a dog to believe they have to be the owner and leader and delegator etc etc.
A German Shepherd Dog is lying in wait to get your checkbook and car keys. You will NEVER get that out of them but they will not act on it if they are balanced unless you are a weak energy follower and need THEM to lead you around through life. hahaha.
That's all I'll say about that.