Peterbilt 379 vibrating .need help !please

I have a Peterbilt 379 short hood 1997 is that a low air leaf ? And yes I feel it on my seat . Don’t feel nothing on the steering wheel

 

So I took off both drive lines from tranny to first differential . I changed u joints but I just threw drive line back on . I didn’t mArk it before I took it off . How do I check my u joins are line up ?

 

I don’t feel it in the front or steering wheel

 

Provide vin and we can tell you for sure but is most likely is low air leaf. This is type of vibration is most often an issue in the driveline.

Was the vibration present before the u joint changes? Or only after. Your best bet if its outta phase and or outta balance is to remove both pieces and have them balanced and reinstall. I have chased phasing issues before and often the fastest route is to just rebalance the whole thing. Always mark yokes and sections before removal and put them back the same during servicing.

 

This will give you an idea. It has to be in phase . You do this with the splines on your slip joint

 

I am kinda new here but the wearing out of ur tires could be the tires causing the vibration. An alignment and fresh tires of course will help. I had a problem, same thing, in neutral and around 50km/h , vibrating light that. Good day