If you just drain all the old fluid and put new fluid in you should be fine but if you want to be very thorough...Drain all the old fluid. Put about 1/2 to 1 qt of new fluid in and roll the truck up and down the driveway once. - don't drive it, just spin the wheels a little to get the new fluid to "rinse" the inside of the housing and gears. drain again and refill. don't forget to add the really expensive additive if necessary.