All depends on what your doing. if your out playin gin the mud and crap then i would be doing the diffs at least once a year if not more. 30k seems a little early for t case i would do trans and t case every 60 at least. I just changed my trany and diff after about 4 years and it was as clean as teh day i put it in. Imm in the process of changing everything over to synthetic in the trany, t case and diffs. Did the trany and t case and can notice the diff in how much better it shifts and getting a bit better gas millage