1996 4-runner, 6 cyl, 4WD, 166,000 miles, how often to change synthetic oil in diff and trans case?

Mine has 168k miles, and I usually change oil at 5,000 miles. This is one of the few pieces of maintenance that I do not do myself, since you really can't get much of the old oil out without using one of the T-Tech or similar machines. The website that I cite below is great, and I would suggest you check it out. Good luck !