four wheel drive prob

[squirrely_1]

I was trying to pull a friend out with my 2005 Z71 on sunday, and it wouldn't lock into 4 low, wheels were straight, truck was in neutral. tried driving back and forth all the tricks. nothing worked. So i placed it it in 4 high, it locked in, but when i would try to floor it, the truck would just bog down and barely spin tires. i have never really used 4 wheel in this truck for anything but traction, not pulling someone. Is this a feature for self preservation , it won't allow it to get above a certain RPM? Any input would be great, oh and btw i could hear it trying to lock into four low..

~ squirrely