Drain it, and pour in 2 quarts of Diesel fuel and maybe a quart of 90 weight. Run the engine with the trans in neutral for 15 minutes or so and drain it. Virtually all the water contaminaztion should be gone, but the rust damage to the gears will still be there. Since water is heavier than gear oil, the cluster gear would have been partially submerged in water for a long time. The grease and water wouldn't have emulsified until you ran it awhile.
Jerry
