Well, your question makes me think that you have not done this before so...you don't know the status of the fluids currently in the diff and final drive (when they were last changed) so...I'd change them instead of just topping them off!
Use SAE 80W or SAE 80-90W API GL-4 Hypoid gear lube. Since there is no "ye11ow metal" in these gears, you can use GL-5 rated oil. I use the Lucas synthetic.
Also, be aware that the "proper fluid level" for the rear drive unit is something of a nebulous thing and it's generally best to just drain and re-fill with the proper amount unless you have a leak you are trying to compensate for (which you should fix anyway).
If you would like, I can get a copy of the Yami Service manual for your quad but I would suggest getting the Clymer's manual as well...it's a great book.
Mine: '02 660 now in custom Hammered Copper paint
Mods: Custom air box mod for big-o' clamp-on K&N, Dial-A-Jet, 26" BH 2.0's on ITP SS 106's, SuperTrapp IDSX exhaust, Maier fender flares, black Ricochet skids. Remote res. Works A-T Steelers shocks on all 4.
Hers: '04 660 LE (Ruby red)
Mods: same A/F mod, 1.5" billet spacers with BH 2.0's on stock wheels, Yami aluminum skids.
That which doesn't kill you, leaves scars to remind you not to do it again