Can you identify my car's transmission problem?

I have a 2002 Saturn sl2 with 175,000 miles on it. It drives fine on flat roads; upshifts and downshifts fine. However, on an incline, when my needs to downshift (going about 40mph), the car jerks and lurches, like I'm being rear ended and then downshifts. I want to say that the transmission is fine, but there's another problem that I can't diagnose. Any suggestions?