2004 Chevy Silverado low range problem

I was pulling some logs with the truck yesturday,
i put it 4 low and hooked to a log, the back tires was the only ones that spun. The indacator showed it was in 4low. So I put it in 4 high then all 4 tire spun. How is that?
I did drive it fowards and backwards after I put it in 4 low, so the gears could line up.