Welcome to the forum, the services you are inquiring about are easier then an oil change to do yourself.
Note when you are under the truck, each item has a low hole and a top hole, diffs are "should" be rubber. Now, with transfer case, two main things, always take the top nut off first in case you can't, and make sure you do not over torque because transfer cases can fracture if you over torque them. If diffs have bolts, again just take top bolt off first, if it is rubber and bottom is a bolt, whatever best practice top nut comes off first, then simply drain.,
So if you don't buy amsoil severe gear with the bags, you will need a slippery pete type pump, those attach to the quart container, make sure you get the right slippery pete for the correct item you order, if you order gallons you get the gallon size, if quart you get the picture. Once drained you put the bottom bolt back, then fill til it pours through open top bolt, put top bolt back, done.
Now, 900 is sick, I hope for the dealers sake they meant removing diff cover and hyper cleaning with new gaskets etc.
Check you manual, most rams
75w90 or 80w90 or 85w90 for front diff, any will work 75w90 is the most popular
75w140 rear diff
Atf 4 for transfer case, need to verify that for sure
You do not need lsd additive whether you have it or not, all gear fluids now add this regardless unless it is a specialty fluid.
Bottom line, go under truck and verify you can find the two holes on each item.