I have a 2002 suburban 4x4. I have a vibration at high speed. I changed the rear bearings, bought new tires. U joint seem ok, but not sure. I don't hear any squeaks or clunking when shifting into gear. The u joints have a grease fitting. Is it possible that they just need greased or they could be bad too

 

If they have a grease fitting, they have been replaced I would bet. Best way is to just rotate them back and forth by hand. If worn, likely a lot of slop.

I had a worn rear one on my 2007 that had a grease fitting, when I checked it, tons of slop. When I replaced it, one half of the bearings had turned to dust. After replacement, the vibration went away.

 

take the rear shaft off and move the joints back in forth in both directions, if theres excessive slop it should be very noticeable but a lot of times they will be seized or you can feel a catch when moving the joint by hand. Sometimes you can tell with it on the vehicle still but the only way to be sure is to take the driveshaft off.