2004 Mazda 3 Speed Sensor

i normally like to go to google to figure out my car problem before I head to the dealer or a mechanic, but right now I am lost and even more confused. This is what I know about the problem:

This all started when my AT (Automatic Transaxle) light came on, the car did a jump/jolt while I was driving and then shifted from 4th to 3rd gear. (the car is not manual just mazda has the gear numbers from 1-4). When I came to a full stop the light went off but the check engine light came on. Since it didn’t seem like an urgent matter at the moment I sadly didn’t take care of it. I found out a few weeks later that the diagnostic error was a Speed sensor. I kept delaying this until it finally start making a horrible noise when I would put the car in reverse, it felt as if the motor mount was about to let lose. when I had the speed sensor replaced I figured that would be the end of it. It fixed the problem for a quick second until to comeback a day later. Can anyone help? I hope i don’t need a new tranny.