Is is normal to have somewhat excessive vibration at highway speeds? I have drivin a few other vehicals, a chevy astro van, a bmw z3, a chevy van 30, plus a newer nissan murano and my sportie has a butt-numbing vibration. I realize the sportie is not the nissan and not the bmw, but the two vans are much smoother in the vibration department. I'm not talking about the truckie ride, it's vibration. Anyone suggestions?

Wheel balance, wheel bearings, driveline u-joints, bent wheels.

I just changed the front wheel bearings (all of them), and I get the tires balanced and rotated every 3000 miles. So that leaves the driveline u-joints. How many are there, how hard are they to change, how much will that cost?

does it get worse when you hit the brakes?

No

rear drive shaft u-joints x2,rear wheel outer bearings,possiblity also rear differintal input shaft bearing,from what you say this is happening at highway speeds so you could rule out your front wheel bearings if you have had them replaced,also do you have your front hubs unlocked,if not this can place the front drive shaft and axles into rotation also giving some vibrations,hubs should be unlocked unless needed.

Do you feel it in the seat? Rear area
Do you feel it in the steering wheel?

Does it get worse accel or decel?? Driveshaft/Rear Axle

Try jacking it up, placing stands under the frame, and place it in drive and see if it still vibrates, if no vibration come to a complete stop and put it into 4wd.

To check your u-joints grab the driveshaft near the joint and push and pull parallel with the joint. If you see shiny spots or even a ring of rusty powder around the center of the joint where the cap seals are then its a good indicator of improper lubrication and possible need of replacement.

If the vibration is from 45+ it is usually wheel/tire related...

I appreciate the info guys. I haven't had a chance to put it on the lift, but I will give your suggestions a shot.