Hi Christie, may I ask why you think the left hand tension you’re experiencing is caused by the shoulder rest you’re using? I used to use a couple different ones as I was growing up but when I began to play professionally, I really had to check on this issue, because when it came time for shifting into upper positions, I just felt like my violin was not secure and it moved inward as my hand began to travel up the fingerboard. Then I tried the KUN and have used it ever since. I like the kind they originally made, not the collapsible version. I recommend the KUN to ALL my students too on all sizes of violins from about 1/2 size and up.
You can play with angles. Sometimes people with more narrow shoulders need to pull the upper end toward the left, bringing it closer to the person’s neck.
But so many things can cause left hand tension, so it might be a good idea for us to talk this through beyond shoulder rests.