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Q:

have a little vibration while in drive or reverse when im stopped on 1999 GMC Savana 1500

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just had donut gaskets replaced on exhaust manifold. my mechanic seems to think I need tune-up....help
(1) Answer
review when last tuned. maybe he is right about being due for tune up. however if vibration occurs only when in gear than look to the drive train(spec.drive shaft). possible worn U joints. do you hear a clunk or clicking when shift forward or reverse?
also possible worn bushing inside output seal. check for any sepage around output seal(front end of rear drive shaft).
rec. fuel system clean. ethanol treatment to 1 tank premium. these are simple things you can check for yourself.
did he perform a diagnostic run? if not than you should get a diagnostic done to see if any issues you should be aware of.