Vibration at certain speeds

Sounds like it could be a possible drive line angle problem.
The drive shaft angle changes from bob tail to loaded.

 

Several things come to mind. I note you said tires balanced at alignment shop. But was there an alignment (3 axle) done?

Or there could be bearing issues.

You really need a competent mechanic to do a ride along. Often times things can be sorted out quickly when that happens. Or at the very least, it will give a good mechanic a starting point.

 

The fact that it does it loaded or unloaded and coasting or pulling is interesting.

My 97 9300 I just bought has a heck of a vibration you can feel through the shifter.

I know the steers are shot even though they look good.

The vibration from them is getting transferred through the truck and is shaking the motor and transmission.

Thinking my motor and transmission mounts are shot .

Can you feel the vibration change loaded unloaded through the shifter?

 

Your issue is in fact in the driveline or angles as @Hulld suggests. Measure and set the suspension ride height to factory specs. From there I would recommend you pull the main drive shaft assembly and have it rebuild and balanced at a reputable driveline shop.

While less likely in the intermediate shaft between the axles, probably not a bad idea to pull that one for a shakedown as well.

 

The rod isn’t adjustable but the arm on ride height valve is. Two bolts 1 stationary 1 slotted. Get proper height info loosen both just enough to move set and tighten. Check axle torque rods ( bushing bad?) Spring bushings.Some Times after replacing a u joint if Drive shaft is installed at 180 degrees off from original locations a new vibration will appear.Have to take driveshaft out and reinstall 180 degrees with new straps and bolts. It’s a long shot.Excessive ride height will cause a chatter in gear shift / trans while accelerating, Excessive low height causes grinding when coasting in gear. Other than Bent rim or Excessive wheel bearing play.

 

And check the motor mounts too