While both might use the 700R4 transmission, the diesel engine produces significantly more torque at lower RPMs than the gasoline engine. To make effective use of that torque and achieve reasonable fuel economy, the diesel truck would likely have numerically *lower* axle gear ratios (e.g., 3.73 or 4.10) compared to the gasoline truck (which might have 3.08 or 3.42). Lower numerical ratios mean the engine doesn't have to work as hard at highway speeds. The gasoline engine needs higher numerical gearing to provide adequate acceleration.
Therefore, you cannot assume the same gear ratio just because they share the same transmission type and are both 4x4 half-tons. You'd need to check the specifications for each specific truck's axle to know for sure. This information can often be found on a sticker on the driver's side doorjamb or in the owner's manual (if available).
