Engine Vibration

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When the engine is idleing it runs fine but when you excellorate a bit it vibrates between 1000 - 1200 rpm. as the excellorates more it tends to go away. Every time u shift gears it does it the same rpm range. the truck is 1998 Mack rb model tandem dump. I hope that some one can tell me what the problen could be. Has anyone come across the similar situation.