Of course you need to stay below 50mph in 4WD, but that isn't such a bad idea anyway.
Is being below 50mph in 4WD in your owners manual? or just your practice?
My 92 toytruck says right in the manual to stay at 50 or slower in 4x, but I have looked in the manual for my 99 4runner and can't find anywhere it says to stay under 50 mph (of course if the road conditions are bad enough to warrant 4x4 you shouldn't be going over 50 anyway)