why does my drive shaft vibrate?

Hello, well, 90 miles per hour is kind of an excessive speed -- and will be very hard for a law abiding technician to safely and legally recreate for the purpose of diagnostic testing. As a result, it's mostly "theory" here -- Things that may cause a high speed (only) vibration: -wheel balancing, including road force balancing needed -Axle shaft out of round / balance -Drive train issues, such as with internal components of transmission and transfer case That's really about it. Hands on testing is needed, but it may not be possible to duplicate your specific concern, making validating repairs difficult also. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops