I have two very low mileage 1994 trucks both with under 50k on them. 4x4 automatics regular cab. 1 st and 1 slt. For some reason they run rougher than my 1995 2500 4x4 slt and my 1995 3500 dump. There is more vibration, noise and actual shaking in the 1994 trucks which are in way better condition with low miles. Any ideas why??