The manual calls for plain Mercon ATF. I have always used Mobil 1 ATF with no additives in all my trucks t-cases and it is fine.
The manual tells you to change it every 60k but it will look like tar by 30k. I change mine every 30k. It is easy to do yourself if you get a suction gun to fill it. Some Rangers have a round vibration damper covering up the drain plug, there are three bolts on the bracket to undo and it comes right off. The case is aluminum so don't over tighten the plugs or you will strip the threads. Just snug is fine.
If the lube in your 98 has never been changed (likely) and it has over 100k on it (likely) it may drain out like molasses on a winter day. Don't panic, but fill it, drive it 3000 miles or so, and change it again. Just to clean out the crud.
Late-model manual trannys call for Mercon ATF also.
L/S rears call for 80w-140, most of the synthetic axle lubes are 75w-140 which is fine. The front takes 75w-90 or 80w-90...know that on a 98-up the ONLY way to change the front diff lube without pulling the entire diff out of the truck is to take it to a quicky-lube (I go to Valvoline) where they drain it with a suction machine. Because there is about 3/8" clearance between the diff cover and the crossmember you will NOT get the diff cover off. Costs about $49, it's worth it.