Strange Vibration when loaded

If you have 379 Pete with Low air leaf or Flex air suspension your laden ride height is 8.5" or 216 mm and uladen air ride height is 8.75" or 222 mm.
Air Leaf 12" or 305mm unladen and 11.70" or 297 mm laden.
Air Trac 11.38" or 289 mm uladen and 11.00" or 279 mm laden.
Low Low Air Leaf 6.75" or 171 mm uladen and 6.5" or 165 laden.
To check your air ride hight you need to measure the distance between the bottom of your frame rail to the center of your drive axle.

 

You want to inspect the rear suspension for anything broken or worn allowing excessive movement, set the ride height, properly. After you drive for a while, feel the shocks if they are cold they are junk. Sounds to me you have a drive line angle problem and it needs to be recorded and run threw a program Alison and Eaton has great programs. What ever you do get it fixed that vibration will damage the transmission and even if it has warranty, the damage will tell the mechanic the problem and it will not be covered.
Check the drive-line Phasing!

 

Phasing shouldnt be a problem unless the driveline has been removed and reinstalled should it?

 

You do not replace parts unless there is a problem with that part, and if the "U" joint cup is loose in the yoke you will need a new yoke. That "U" joint you do replace.
I have seen lots of trucks come in and have the drive shaft out of Phase.
I found one on a brand new inner axle shaft.
A local pebble hauler drove around for 6 months with his main shaft out of Phase.
There is a reason they put erasers on pencils.
The yoke may be the cause of the vibration or it could be the effect. Take it to a decent shop and get them to run the numbers. It should cost you no more than an hour to know if there is a problem more if they find problems. A vibration costs you a lot of fuel mileage.

 

Mine did that with a brand new truck. It just come out of the factory improperly adjusted. I went in the PB shop and he fixed it in about 15 minutes. It amazed me something so small could cause so much problems. But like John said, if it's something else he can tell you quick.

 

Do you know if those are the same measurements for a KW AG380 which is the same thing as a pete low leaf air?

 

I do not know if they are the same. I own a 379 and I got the info I posted from a Pete dealer.

 

still bet its them drive rubbers . . . . . . and there is something you aint seeing.

 

A front end tow issue can cause odd vibrations that you drive in and out of at varying speed ranges so I've been told .

How do you get your drive-line checked for phase ?