When should service be performed on transfer case and the front and rear diff.?
In my opinion, this should be done at least every 60k miles.
However, my opinion is not necessarily correct. If you refer to the mfr’s maintenance schedule, that will give you the correct answer.
The maintenance schedule that came with the vehicle should have the answer to this question.
If you have over 60K miles you should consider changing these fluids. Do you pull any trailers? If yes, the heavier they are and the more miles you trail them the faster you need to change these fluids.
Find the manual and consult it. If you trailer, then follow the “severe” schedule. I have an '01 Toyota Sequoia that is very confusing on the rear differential. The book says every 5,000 miles to change the rear diff fluid if you tow a trailer. Every Toyota dealer I ask says this isn’t necessary. So, sometimes you have to make your own decision.
60K if you are fussy, 100K if you are not…Magnetic plugs are worthwhile…