I know there are few threads related to this but haven’t been able to find a concrete solution or direction to address my issue.
2020 ford f150 5.0 10 speed. 66k miles
2wd

issue: when driving the truck it shifts fine but when slowing down around 6th gear around 25 mph I get a vibration/rumble. I don’t get it in any other gear or downshifting, I’ve driven the vehicle and put it on N when slowing down and it doesn’t do it it brakes fine no rumble/vibration. If I also brake really fast so that it doesn’t go through 6 gear it doesn’t do it. Only if I coast and slow down
I’ve resurfaced rotors, I’ve even replaced the transmission with a new one brand new one and new torque converter, issue still persist
Replace the rear axle fluid - issue persist
road forced and balance wheels and tires - issue still persist.
inspected u joints and look good
drive shaft looks good.
even made the update to the pcm - issue still persist.
not sure what else it could be. Any help greatly appreciate it

 

Seems you have ruled about everything out except the front drive shaft spline into the rear of the trans.
I would be taking the drive shaft out and Lubing the front Splines to see if the issue goes away.
The issue , I think, would be due to the rear axle rotating downward under Deceleration that will move the front Spline into or outward on the Trans spline and cause sticking due to being dry of Lube.
It maybe that the rotational speed just happens to make the sticking pronounced in that gear.
If you do this, be sure to replace the drive shaft in the same location it was removed from for balance reasons. Mark the rear location on both flanges.
The front should go back in the same as it came out.
Worth a try.
Good luck.