Whats the service interval for gearbox, T/Box, diffs??

Just doing me oil/filter change (which I do every 6 months) wondered how often I should do my other fluids it will be 12 months since they were done, but less than 6K (approx 5 actually)

Got enough ATF to do the gearbox, but would need to get more EP90.

levels are all fine, I check them every other month, or before a long trip.

I do diffs, gearbox, CV joints & prop shafts every year with the transfer case every two. My milage is very similar to yours most years with the max of 6000ish very occasionally and only if I 'do' abroad. Have not had any problems or noted wear so far.