Sorry to hear about that, seeing your left arm hurts more than the other maybe you have a pinched nerve. I would go to a chiropractor and get an adjustment. With our industry our backs take a beating, even if we're not landscaping. Anyways...my shoulders hurt at times, but usually after a couple hours of blowing. I think you're on the right track, somehow you need to support the weight in spots other than a shoulder.
Let us know if you figure something out.