I would disconnect the battery for 12 hours (overnight) and reconnect it. This will let the TCM relearn the trans. If that does not help, add some lucas trans fluid additive. It sounds like the seals in your trans are old and worn causing valves to stick.

 

It feels like I am being rear ended.

If you were being rear-ended, you would have a sudden forward acceleration and your head would snap backwards. On the other hand, if you suddenly decelerated (like slamming on the brakes or being hit head on) your head would want to move forward. Is this downshift like the brakes being suddenly applied or is it like sudden acceleration?

 

Usually its deceleration as the trans momentarily goes into 2 and 1 at the same time, acts like locking up the trans. Most people say it feels like getting hit because the sensation is the same, smoothly slowing down then a quick slam then back to smooth.

 

This could also be in the solenoid pack. Many problems can be solved by re-building or replacing this.

 

Thanks for the responses. i will give the solenoid pack a try.