General Fluid and matienance questions

[phatal1320]

just got this 95 tahoe with 179k for cheap wanted to switch to mobil 1 synthetic just wanna make sure it wont hurt anything to switch over? and what fluid do i wanna run in my T-case? atf or gear oil? and differentials? 75-90w? Also when do you have to usually have to swap out the timing chains on these? its a 350 TBI. and i also dont tow anything, and my ABS light is on 24/7 i guess the guy said its been on for about a year and the rotors are warped too shit... wonder if that has anything to do with it? or needs new abs sensor?

Thanks.