Strange Vibration

I have 2000 4x4 z71... At highway speeds (60+) i'm getting a vibration that comes & goes. It'll last about 5 to 8 secs than go away for 5 to 8 secs & than come back. It'll do it the whole time above 60 mph, & sometimes seems worse than other times.
Thanks for any help/ideas

What is the mileage on the vehicle?
Many things could attribute to you situation.
Dry slip joint on drive shaft
Worn or dry U-Joints
Worn or dry front wheel hub bearing asm
Un-balanced tires or irregular wear on tires

it has 132k about 56k that i've put on it. Alil while back i had the belts replaced & the mechanic said something about the "harmonic ballencer" looking worn or something to that effect. don't know what it is? or what it's for?
Could that be it?

The harmonic balancer is the hub mounted to the front of the crankshaft for the sole purpose of balancing the crank and dampening the the vibrations in the engine caused by the rotating asm. Normally when this is bad you would feel the imbalance through out the rpm range when just running the engine. This imbalance theroy could be the root of your problem, however you specified speed as a culprit for you situation which tends to make me think there is a drive line or chassis problem.

First of all, thank you todd for your advice/help
Second, how should i go about figuring out where the/what the vibration is coming from?

If I'm not mistaken you should have a tachometer in your instrument cluster. When traveling at 60+ miles an hour and your vibration starts, observe the RPM's. Then when you have a chance, In park, rev the engine to that RPM and see if the vibration is there. If so, your Mechanic may be correct and you will need to replaec the balancer.
Next, Try to isolate your vibration, Is it in the steering wheel or in the seat of you pants or in the back. (I'm not trying to get smart here).
Jack the truck up and see if the front wheel/hub bearings feel stiff. Look for signs of a wheel weight being ejected.
Check underneath the vehicle and inspect the drive line. Look for rust spots around the U-joint caps. see if they feel loose.
Jack the vehicle up rear, end only, use safety stands, and run the vehicle up to speed to see if in fact the drive line is the problem.
This should be a good start. Post back and we will continue. Also continue to monitor this post as many guys will help as well.

I'll do some do some looking around today. Gonna be under it anyways, gotta change the trans filter.
Oh the vibration is more in the seat of my pants (as you put it lol) but do feel some in the steering wheel too.
Apprecaite all the help & i'll let you know what happens
Thnx again