Yeah, what the other guys said.
Hard tellin' what it could be, but I'm curious how many miles you have on the truck. Do you feel the vibration primarily though steering wheel, or through your butt and feet? Besides mileage, how long has it been since you've had a good laser, all wheel alignment? Without proper toe-in, you can get the vibes.
If the mileage is nearing 750K +, it might be time to rebush the truck, especially the front end, and have the kingpins replaced. It's not cheap and I've had it done to my FLD120. It is cheaper the sooner you catch the wear as the complete spring hangers are more costly.
Shocks? I agree that after about 100k they are pretty worn but I've tried the high dollar gas jobs and the cheaper ones. Not much difference in my opinion. If you suspect shocks, drive the truck for a while, preferably over roads that will work the shocks, and feel them up. If they are somewhat warmer to the touch than nearby frame metal, they are probably working OK. If they are the same temperature, they aren't doing their job.
If you've had the rotation and balancing done with no improvement, I'm inclined to think the problem is elsewhere if the tires aren't worn in unusual patterns.
The fact that you get the vibration a a specific speed range leads me to something else I've experienced twice in the past few years. That is the carrier bearing in the middle of the driveshaft.
Crawl under the truck and push up on the driveshaft near that center bearing. Did it move? Are the bolts tight or is there slop between themounting and the frame rail? This inspection takes all of two minutes and is free.
Vibration in that bearing usually comes in at a certain speed range and is felt more through your seat and feet that through the steering wheel, although some does come trhough the wheel. The first time this happened to me at around 750K miles, simply tightening the two bolts solved the vibration problem.
The problem returned at around 1,200,000 miles (yes, I'm cheap) and this time the hard rubber mounting surrounding the bearing had simply given up the ghost. the new replacement runs about $51 at a good dealer or $65 at a stealer. figure an hour labor for the replcement at a good shop. BTW, the newer carried bearing has a few holes ventilating the rubber and seems to work just fine.
Hope this helps.
Where is Torkwrench when we need him? =0)