Do the front differential and rear need to have same gear ratio 1988 Chevy silverado 4x4?

Yes, the front and rear differentials in a 1988 Chevy Silverado 4x4 must have the same gear ratio. If they don't, you'll experience drivetrain binding and potential damage when driving in 4WD. The different ratios will cause the axles to rotate at different speeds, putting stress on the drivetrain components.