What causes a 2002 Subaru Outback to jerk at different speeds?

The description that you provided gives me little clue about the circumstances when you experience this jerking. You did not mention a check engine light. The only thing I can think of is an engine misfire which should also set a check engine light.

I have also owned Subarus that have quite a bit of driveline "slop". That is when going from decelerating to accelerating, or accelerating to decelerating, there is some looseness in various components that makes the transition less than smooth. This would be considered normal and is not a problem that you can fix without a lot of expense.