I have a 97 F150 with a 4.6. I have a vibration when I'm going between 45 to about 60 and then it goes away when I get over 60. And some times when I slow down and cruise about 30 to 40 I feels like something is out of balance but not always. I had the tires balanced and I checked the U joints and they seemed fine. I replaced them about 9 months ago. When I moved the drive line up and down there was a little bit of slop where the drive line slides into the transfer case. I wasn't sure if this was normal or is my transfer case going out. Somebody help.

 

ive noticed majority of my vibrations are from the idler arm backing the bolts out of the frame or the tierod ends are wore.
but are u saying the slip yoke has play on the shaft? thats no good. mine has some play in the splines but not much.

 

I have something similar going on in my truck, it makes a harsh vibration/almost grinding/rubbing sound off and on when I am driving. I replaced the U Joints, checked all fluids, etc. IDK what it is, but when I get out after driving it, something like a fluid smells like it has gotten really hot and is burning.

Let me know what is wrong with yours if you figure it out. Maybe it is something similar.

 

Its not alot of play, but there is a little.