Gravel, snow, ice, sand, mud... anything that is a low-traction surface so that the drivetrain can slip a bit to prevent binding. As for the speed, if conditions are such that you need 4wd engaged, you have no business going over 30 anyway. If you can comfortably go over 30, then things obviously aren't bad enough to need 4WD.

You're truck doesn't say anything about when to and not to use 4wd? Mine came with a manual specific to proper use of the 4wd system. A 'Cliffs Notes' version of said manual is also printed on the part of the sun visor that faces me when its down.