just have the rear shaft in ...gasket is moving freely..jeep vibrates..must know

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only have the rear shaft connected..went offroad a while back i think i moved the washer for the rear shaft for the gear box ...jeep vibrates alot on excellerating and uphills ! whats the problem ...i think its not the clutch ! might be something to do with the gasket! but not sure? how do i check to find out?