Replace neway air beam with new suspension

An acquaintance of mine had that suspension on a mid 90's western star too. He never had too many issues other than parts availability like you said. Are the frame rails parallel to the ground? Are the shocks the right ones for that suspension? Sometimes these heavy spec suspensions had specific shocks. Other ones will fit but not be right. I would probably go to a stiffer shock to dampen out the hop. Or even test drive it with the shock disconnected to see if it

 

Yeah, I know. I've heard the "all new tires" story before. I'm telling you, if the bounce or shake is sustained, it is a tire or wheel! Tires can be mis-mounted to the rim, and the wheel can be slightly off center on the hub itself and therefore the assembly is running out of round as it rolls down the road causing your problem. You need to place both drive axles up on jack stands and run this while someone else watches the drives tires. I bet you will find one or more that are going up and down as they spin.

 

Try a slower speed. At 60 those wheels are just a blur. Also, please tell me you aren't running recaps!

 

I do no

I do not know anything about that suspension but will share a couple of things.
I cannot recall this guys handle but I thank he has a red mustang avitar. Made a good point to knock all the axles out and run it with the P/D locked in. That may answer the driveline or tire and wheel question.
If you jack up the drive tires one axle end at a time look at the run out from a distance. Do not twist it up because the power divider spider gears double the speed and the differential carrier spider gears double it again. High gear at 40 MPH on the speedo equals 160 MPH at the tires. Like HD said keep it slow and the gear drone is normal.
One example of that is a 9670 with 500,000 miles transfered in. Thought we had it narrowed it down to one wheel set out of round a bitt. Swapped tires, and rims. Then sent the hub to a machine shop which said it was perfict .Replaced the bearings, races and reinstalled, same problem. Swapped the hub with one off a truck on the for sale line. Problem solved and I hope you did not buy that truck.
I still find it amazing how things like this can change so much with road speed.

 

I agree with the others, a sustained bounce is something out of balance. Did you check your u-joints? Apples to oranges but I had a pickup truck once that had a very bad and constant hop at a couple specific speeds. Felt like someone was rapidly kicking the bottom of the seats. Found a u-joint had a missing cap. Believe it or not I ran it like that for over 5,000km like that before I changed it lol. Once I changed it, hop was gone.