If you are asking how long it will take a shop it may not matter. Many shops charge by the book, so no matter how long it takes, changing the rear diff may be a 1 hour labor charge, even if it takes 15 minutes. With a lift and all the proper air tools and some experience the diffs could easily be changed in under an hour, this is for both assuming 4wd. You do have to remove the front skid plate to get the front diff. The coolant flush would take about a half hour and the spark plugs about the same, you may as well have a new PCV installed at the same time. If I did all that in my driveway it would take me no more than 4 hours, and I am also adding in the transfer case as well. If you have a 2wd then you are looking at 2 hours of work, but a Toyota dealer will probably chanrge a lot more.