Check your converter they are known to can out seperate That happened to me in my 1998 Ford Explorer - turned out to be the differential. It started out as a loud thump, thump, thump, then just got louder until it sounded as though something under the vehicle was being dragged and grinded. It was incredibly loud! This all happened in a matter of about 15 minutes.
possibly your wheel bearing is on its way out
Could be a small thing like wheels out of balance 40+ "Noisy Inside Car" bad wheel bearings 30+ mph also decelerating harder to control "Steering wheel wobble "on accelerating / decelerating" "Noisy Inside Car" steering linkage: causes the steering wheel to wobble at 40+ mph decelerating harder to control "Steering wheel wobble "on accelerating / decelerating" My brothers car had the above problems I changed his rear spindle, outer, tie rods, "both were shot" bad bearing "blown bearing"
could be a center support bearing or a bad engine mount. i would check those first.
An accelerating force is the force which causes accelerated motion.
It sounds like you have some steering linkage loose, ie outer tie rod or other front end wheel component that is loose or bad
It seems as if your car is low on or out of motor oil. Having an oil change done should solve the problem.
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.
Lack of propulsion.
It is the alternator....