If the present trans fluid looks pretty good, you could do a single drain/fill. That gets you about 40%+ new fluid. many folks will then drive a little, do it again, drive a little, do that a 3rd time. 3 d-f cycles get you over 80% fresh fluid. Or, do it once, maybe again next year, then do it every 2 years...? w'ver you're comfortable with. The tourque conv. holds a lot that won't reach the drain plug. There are slightly more complicated ways to to do a full change. Remeber to shift thru each gear after top-up, and check while IDLING on level ground. Also. the marks on the stick are only a few ounces apart - NOT a qt like on the eng. oil dipstick so, fill slowly. Also, fill the front diff slowly - I use a funnel that has a hose attached.
For the front and rear diffs - you might check into a WalMart semi-synthetic/synth'blend' GL-5, affordable and reports are it's OK fluid. If you do the rear diff, take the FILL plug out first. Don't want to drain it and then have no way to move/drive it if the fill plug is stuck. I had to use my floor jack on a breaker-bar handle. A gear oil pump will help fill it. some cars go to their grave without getting diffs serviced. But, if you do it, probably never have to do it again.