Vibration 55-60

Before you got empty valet changing parts go get cheap gopro camera. Attach some small strong magnets to the camera casing base with epoxy putty. Put this camera on record looking on every drive line component. Sound recording is important too. Run short distance with speed vibration is the most. Review the recordings.
You can hear rattling U-joints, you can see vibrating parts.
Ride height valve need adjusting if front springs are flattered. Not many mechanics know how to set Volvo ride height.
First step is set valve to have recommended height.
Final step is adjusting valve to have required pinion and drive line angles. For quick check drop the trailer, let truck 10 minutes for airbag stabilizing. Put strait bar across frame on the drives. Measure the distance between bar and frame on first drive and rear. If there is more then 1/2 difference the drive lines angles are out.

 

Snailexpress, i have seen an air suspension diagram shows a check valve or one way valve on suspension supply line before reaching the leveling valve , which one is it? the black plastic air manifold inside the frame left side with multiple air lines in and out ,plz ?

Road Ranger, if you didn't fixt yet, this pdf page 17, suspension related causes for vibration.
http://www.hhrvresource.com/VolvoDocs/60-79/PV776-TSP27725-1.pdfhttp://www.hhrvresource.com/VolvoDocs/60-79/PV776-TSP27725-1.pdf

 

Sorry haven't seen your question. Yes you are right about manifold.

 

I had a similar vibration in my truck. 55-60mph regardless what gear I was in. I wanted to avoid shops so I replaced the u joints and midshaft bearing myself. Vibration was still there. All looked good except slip joint had a little play so I said screw it. I took driveshaft back down and brought it to a shop and told him to rebuild it and balance it. Both tubes were bent and had to be straightened. Also welded in a new slip joint. There is no way to know unless you put it on the balancer. Vibration gone completely now. Also I was told that the front shaft is balanced as one piece. If you remove the shaft between bearing and front differential it needs to be marked and reassembled the same way or it will affect balance.

 

id have the drive shafts checked by a local shop. if you dont know one, call the closest volvo dealer and ask them where they take their drive shafts to get checked at. they will also check your carrier bearing. it shouldnt cost MORE than 400 but is usually 75-100 per axle for a balance. they can also eliminate the carrier bearing since they check the first and second drive shafts together