balance tires
Tires or rims out of balanceBent wheel
There are several problems that could cause your car to jerk when slowing down. The most common cause would be a transmission problem.
One tire has a belt separation.
If you are driving for a period of time over 45 mph then use overdrive. If you are driving under 45 mph or in traffic that is speeding up and slowing down over and under 45 mph then do not use.
3 HOURS WITH NO STOPS AND NOT SLOWING DOWN
Need more info. Does just the engine run rough or does the steering shake or does the whole vehicle shake betwen 50 and 55 mph?
Yes, negative acceleration is slowing down. Take the example of a car going from 0 to 60 mph this would be positive acceleration. If once it reaches 60 mph the foot is removed from the petal the car will continue to move but due to friction and wind resistance it will begin to slow down, while slowing down it is in negative acceleration.
Wheel balance
It might be your transmition...
If the rotors are warped it can cause a shake when you hit the brakes.
If you are getting a very harsh downshift, take it to a shop (dealer) with the ability to update the software for the transmission computer.
Check or replace axles, worn axles are primary source of shaking when accelerating.