2001 international 9400i vibration problems

my truck was doing something similar to yours and it turned out to be the crush washers in the front drive carrier had broke apart and the gears were out of alignment and were grinding each other apart. I had to rebuild the rearend because the gears had alot of damaged teeth.

 

Have you checked your ride height? That will throw your drive line angle off, sure enough feeling like a vibration if only slightly off.

 

I have a 2003 model same truck IHC 9400i now that's doing that but the cause is the slip-shaft behind the carrier bearing. Getting ready to have it rebuilt with new urethane kit. Rowe Truck Equipment said he'll do it for $600 which includes new u-joint. Before that I had a IHC 9200i where the shaking actually was coming from steer axle. Unbalanced cheap brake drums on steer axle was throwing that baby everywhere and difficult to control. The Volvo I had before that had a failed carrier bearing. When doing a brake job for example, spend the extra money and get the balanced brake drums regardless of which wheel end it goes on because it makes a world of difference. As for drive-line problems in reality they are not too expensive to repair. One other area you should check closely is the mating of the transmission to back of engine because I had a Peterbilt and the upper bolts fell out and transmission was hanging down causing vibration. The only thing that kept tranny from falling to ground was the rear frame suppports. At the top of bell housing mating to engine, I could stick my fingers in there. This in return damaged not only the clutch assembly but also the flywheel as well. And last but not least is pinion angle. Check that drive axle suspension is where it should be because for airbag suspensions, too high or too low will cause parasitic vibration in drive shaft due to excessive pinion angle. Those are four items to put on your list of things to check. After 30-years in trucking, I've learned a thing or two about trucks and truck maintenance.