Lots of things can cause vibrations.  Of course tire/wheel balancing is
the most common.  What about drum/disc out of balance?  Drive shaft,
flywheel or harmonic balancer?  Bad u-joints and many other things.

I have a 2003 Ram, with the Off Road package (I hate the factory 20"
wheels) and don't really have any vibration, except when the tires are
stone cold after it sits out in the lot at work, then it has one for a
couple of miles. It has a fair amount at 45-50MPH

I drive the same way home almost every day and I just thought it was the
pitiful roads, but I forgot something at work one morning and went back
and then headed home again on the same road and noticed it was a lot
smoother and there wasn't nearly as much tire noise as there was when I
would drive down the road with cold tires.

On the other hand..

I had an odd shake in a car I used to have, and the dealer repeatedly
tried to find it. Finally, the pinion seal on the rear end started
leaking, and they replaced nearly everything in the rear, except the
axles. I don't remember what it cost me, the car was out of warranty.
It started leaking a month or so later, and got bad pretty quickly. The
shake wasn't changed a bit anyway, and before I could get it back into
the shop, it started making noises like a bad U joint, and it was bad.
They replaced both U joints, and the guts of the rear again, free, and
it STILL shook. The pinion started oozing again, and I went to a four
wheel drive place that made driveshafts and built rear ends. The owner
said almost instantly, "I bet the driveshaft is bent a little, or is out
of balance, or both".

He was right! It was bent a tiny bit, and was not very well balanced in
the first place. They had a strobe light thing that froze the shaft
motion and it was plainly off. Not a huge amount, but enough to tear up
the pinion, and the rear U joint got beat on too.

They cut me a new shaft, and balanced it to some insane speed and off I
went. It was better than it ever was, nice and smooth.

I took the car, and my friend brought the old shaft to the dealer to see
what they would do about the still leaking rear. The service manager
drove it, and said, "Yep, that must have been it!" I was shocked,
amazed, floored when they replaced the axle guts again, for free. It was
the only really hassle free dealings I ever had with them.

The place that made the shaft and the service manager both guessed it
was caused by hitting speedbumps. There were several huge ones in
shopping centers near my house. The shaft had a tiny shiny spot right
where the bend was. Maybe the size of a dime.

BDK