Do you mean Tune? or a Tune Up? If you mean tune, waite for serious mods like a blower. For a tune up? That really varies. If you have platinum plugs, they are gook for a 100k miles. There is really not much to do on cars much these days, but here is what I do.
Coolant flush - 30k to 40k miles
Fuel filter - 40k - 50k miles
and that's about it.
Regarding gears, 3.73 on a car originally equipped with 2.73 is about the same as having 4.10's on a car originally with 3.27's. Please keep that in mind when reading comments of other drivers.
On a 99+, go with 4.10's PERIOD. Unless, you plan on adding a power adder later, then you may want 3.73's.