The "recommended" top speed in 4wd is 62mph, but it's a selectable part-time system. That means it's meant for highway use, unlike the old part-time systems.
If you tried shifting INTO 4wd at that speed it wouldn't be happy with you (and would hopefully not engage). Top speed I've been able to engage/disengage at is about 50-55mph.
If you had the center locked at that speed (or for ANY length of on-road driving) you would notice the tires "scrubbing" around corners and could do some damage.
The system was intended to be driven at speed, on-road in 4WD so long as the diff is not locked.
You're fine.
-G