I was hoping someone else might have an answer to your problem, because I am somewhat mystified. For us, Grasp Sparrow's Tail is merely one of many postures and is probably much less demanding on the back than most. I can't imagine why you would have a problem with this posture and not with others in the form.
I find that doing the form has a short-term neutral effect on my back whenever I am flirting with back pain; however, I believe the long-term effect is good. I find that doing push hands circles has a short-term positive effect, whenever I have had a little back pain.