The X has a very straightforward 4wd system. It's part-time only, which means that you shouldn't use it on dry pavement, should probably think twice about using it in the rain, and as mentioned, don't go over 63/4/5 in 4hi.
If you use 4wd it on dry pavement, you're going to experience a very nasty feeling when you turn. You'll feel your driveline bunching up, giving you a "hopping" feeling.
And needless to say, don't engage the 4lo or the locker on dry pavement.
I agree with others here: if anyone really needs to be in 4wd, then they shouldn't be driving at 75mph.
Your 4Runner, and many AWD cars, all have more sophisticated systems that are capable of running 24/7. In this case, "sophisticated" doesn't necessarily mean more capable. But they do give you more latitude in deciding to keep it engaged or not.