There's a lot of hype to oils. You'd be best to run synthetic oil everywhere, from the engine, transfer case, to the differentials. If your engine eats more than a quart of oil every 2500 miles, I'd advise not using synthetic, as it won't be as cost effective but can still be considered an option.
96K is nothing to be alarmed about should you decide to go synthetic. If you think about it, a lot of premium cars built today, from BMW, Lexus to Ferrari and Porsche, are all shipped from the factory using synthetic oils.
Mobil1 is the top selling synthetic oil, only due in part to a lot of advertising and endorsements. Personally, I prefer Amsoil first, and will use Mobil1 only if I have problems finding Amsoil fluids (only once every 5 years!).
Stay away from stop leak, and 'performance' oriented engine additives. They generally thicken the oil which can hamper performance and reduce life. For the cost to mile ratio, synthetic wins the price game.
As an example, I own a 2000 Dodge Durango R/T (AWD). I run Amsoil 0W30 synthetic oil all year round. In fact, I only change it once a year and I average 13-18,000miles a year. The oil and filter are both rated for up to 25K, and Amsoil offers a warranty; they'll repair or replace any defective component if it's determined that the oil was the cause of failure. So really, you have nothing to lose here. Mobil1 offers synthetics up to 15K, which is still a long time, and again, if you drive 12K a year or less, either way is the way to go.
If you take long drives (2250miles one way for example), you don't have to worry about changing the oil mid-trip. It's an added convenience. Besides, convention oil changes run upwards of $40 here for no-name oil and filter, and a kid that does the work. Times that by 4-5 oil changes a year and that's at least $160. Amsoil oil and filter (at dealer rate) runs no more than $75 for most vehicles (5 quarts of oil) and is good for up to 25K. I cheat a bit with my Durango, and add half a quart every 3.5K, that's mostly due in part to towing where the engine sees 2500-3500rpm on a regular basis cruising the highway.
Onto differentials, I'd stick with a standard 75W90 oil. your owners manual might suggest using a 75W140 weight in the rear axle, or both, but you'll do fine with the 75W90. It's a lower cost synthetic all-purpose oil. I run Amsoil Severe Gear 75W90 in my Durango, and change it very 50K, as specified on the bottle. I could technically let it surpass 70K as the oil is still fairly clear when I drain it at 50K. If you have a limited slip differential in the Mountaineer (if you're unsure, check for a sticker in the glovebox that will state your axle ratio and something that says "LSD Equipped" or "Limited-Slip".
Most synthetics state on the bottles that adding an LSD friction modifier (it reduces clutch pack chatter when turning, quick acceleration, etc) isn't necessary. A lot of manufacturers do however sell their own friction modifier. It's generally in 3-5oz containers. I do add one to my rear differential in the Durango only for better piece of mind. I look at it as spending $10 now, or $750 for an axle rebuild should the clutches fail. If the clutches fail irregardless, then it becomes a routine maintenance issue. However, running good, clean oil, will increase component life.
Synthetics flow better in cold weather, hot weather, and get to vital components faster when you start the engine. You could say I'm sold on synthetics. And, I am. I've been running Amsoil products in all my vehicles for the near part of a decade. Just the last two years however I've been doing a one year oil change on my Durango. I was skeptical at first only because I tow, where as I don't tow with my Taurus or G6 and put on around 10-15K/yr. Last year was a good testament of how well synthetic products are made. For the extra cost; figure $9-12/quart for synthetic as compared to $4-5/quart for no name oil in my area (FYI - Canada is 20-30% more expensive than the US).
If you've never changed differential fluid before. I advise using a good, high temp RTV sealant to reinstall the cover. A rubber or plastic gasket isn't necessary, as you'll generally need to use RTV sealant anyway, so why pay more?! Just be sure you cover the ring gear and clean the magnet thoroughly. I know with my Dodge 9.25" axle, it has the notorious c-clip that literally holds the axle together. It's a small piece of metal about half an inch in diameter and not overly thick. These clips are known to break, especially under extreme conditions. I just rolled over 180,000kms today (roughly 112K), and haven't had any issues. About 25-35% of that total mileage includes towing anything from 1200-5500lbs.
As to what oil exactly you should use, is entirely up to you. I'm not a salesman, nor is anyone else on the site (that I know of). I know what WORKS. Do your homework, go to AMSOIL - Synthetic Oil, Motor and Engine Oil, Lubricants, Air Filters, Oil Filters and Greases
and run a search on 'comparative motor oil testing' and you'll get the top synthetic oil brands and all the statistics on how they passed certain tests. Mobil1 beats Amsoil in a few categories, but Amsoil pretty much walks over Mobil1, especially in the 25K mile oil drain interval for engine oil and in several other testing categories. Royal Purple would be my third choice, but just be careful that you DO NOT buy any product that states "Racing" on the bottle, or you'll run into trouble as there are additives missing in their racing products that everyday drivers require to operate efficiently. Again, do your homework, shop around for price and perhaps convenience and you'll be fine.
The only thing Amsoil doesn't have, are gear oil bottles with a spout on the top, making them easier to squeeze in through the differential case fill/check hole. Amsoil does sell a relatively cheap hand pump that fits on their quart bottles that works just fine. I've used mine a few times in the last few years and it still works great for a piece of plastic.
Even if you had 250K on the vehicle and it still ran like a champ, I'd recommebd running synthetics. You might gain better mileage, and even free up a horse or two that would have otherwise been lost due to frictional pumping losses. So either way, it's a win-win for the consumer.