I have been getting some odd vib's arount 30 mph and goes away say at 38mph,tires are wering fine no out of bal or anything,I have been under the truck and it all looks good nothing loose,any clue or sim exp with this ?

Pav

Oh i had around 4800lbs in the box and did not feel it then?? Ony when empty.

 

did you run on any dirt roads lately that might have had a tree banch and or rock fly up and ding the driveline? Anything that rotates could be a contributing factor. Maybe a U-joint starting to go out? Or a loose rim, check you lug nuts. If you don't feel the vib from 38 mph on up to say 65 it's probably related to something that would tend to center itself and the vibration then goes away or is cancelled out by other larger rotating masses, like dampeners, rubber tires deforming at speed, etc.
We run a Blown Alky Dragster and the huge slicks on it hop around at slow speeds and then at 200+ mph it's smooth as silk. The tires grow outward and the outta balance condition is eliminated with the incredible rotating speed factors of centrifical force.
Just a few thoughts.

 

it does stop at 38 to 40 and like I said no tire were and yea it lives in the mountains,not the nice smooth road kind and about every other day I got 4000 to 5000 pounds of scrap steel in it,it get washed when I get the steel off and I do hose inside the wheel wells and around the brakes

Pav

 

I had a similar vibration at lower speeds.Turned out being a u joint.These trucks are notorious for the factory u joints failing.Replaced all mine with Spicers.

 

X2 on a possible u-joint. Sounds similar to the vibration when one of my u-joints went out.