I get a hard shift after passing 10 MPH

Hey, when I start my truck and start driving it runs and shifts fine the problem only occur after riding for a while. If I come to a stoplight and stop and then accelerate I get a hard shift after passing 10 MPH. I replaced the center bearing thinking that could be the reason but it wasn't. I thought about flushing and changing the transmission fluid and filter but I got mixed reviews saying don't change the fluid it would make it worst. I also heard the rear end can cause that issue too. Looking forward to your help