Using 4wd
 

An 85 4x4 S10 blazer is my first 4wd vehicle. Not 100% on somethings.

Does the vehicle have to be stopped or in neutral when engaging 4wd?

What is the difference between 4 high & 4 low? When to use which?

no you dont need to be parked or in neutral, you can use it "on the fly" some people say to 30 mph, some say 50, ive done it up to 55, but i would say 30 is good for 4hi, you do need to be stopped and in neutral for 4low tho

4hi is a higher of course, its for a little bit higher speed 4low is for when u think you might be stuck lol, 4low is a lower gear setting giving you more power to try to get out of places if your stuck, i hardly touch it but when i do i usually crawl out

"On the fly" was what I was thinking. A couple of buddies who are Jeep guys was telling me some conflicting things. I know every time I have driven their Jeeps I just throw it in 4hi whenever I needed it without doing anything in particular.