I hope this may help others trying to resolve the mystery...
Had the dealership do a transmission flush for me a couple months ago. All was well then weeks go by and my truck all of a sudden starts to slowly develop a vibration/shudder between 45-50mph but once I pass that speed range the vibration stops and truck drives smooth. I noticed the vibration was more noticeable when my truck was cold and less after driving for a while.

Did a lot of research and thought since I'm rolling on 35' MT my tires may be the issue. Had Discount Tires rotate and balance my tires but that didn't help. Then I thought my U-joints were bad but I checked them out on the lift and they were good.

Finally, talked to my mechanic friend, who bought my old FX4 from me and he suggest might be the trans. Said the dealer flushing that and changing fluid could have caused all the gunk to do whatever and it was having trouble shifting gears from 3rd to 4th. Long story short, bought this stuff and added it to my transmission and now no more vibration! I'm very happy, cost me $6 and solve.

 

Where'd you get it at?

 

Amazon. It was worth a shot for only $6. Best $6 spent on my truck. Takes about 50 miles of driving to start working.

Add it to your trans, just poor it in there.

 

Didn't read your post fully...

 

Just bought some. $6.00 shipped. Worth a shot for that price!