1. Yes you are right 30/35. Unless the previous owner installed a limited slip or locker nothing but gear oil has to be added. this should help
Jeep Axle Differential Fluid Change Service
2.it's up to you if you want to change the tranny fluid but you might as well, if it's a manual it's easy. Drain it and refill it until fluid starts to ooze out of the fill hole. T-case is also easy, drain it and refill it with a good quality atf +4 again until it begins to ooze out of the fill hole, make sure you do all of this on level ground. this should help depending on your tranny and t-case but it will give you an idea.
Jeep NV3550 Transmission & NP231 Transfer Case Service
3. as for the idle could be many things but I will give you a starting point. Can you feel it under your feet on the drivers side floor? if so it is most likley the shift linkage. A very well known problem, I fixed mine at 40k so if you made it 112k your doing well. It was also a chrysler TSB. part's are $20 and it's a very simple fix. Another link to show you exactly what to do.
Jeep TJ Transfer Case Shifter Knocking TSB
All generic info, if you need specifics provide your tranny type, t-case and engine etc... but that should get you started.