According to what I read about my '88 (it might be completely different than yours) I can shift into/out of 4hi at any speed, but 4lo you have to be stopped and in park. It won't go into 4lo on mine even in neutral...

I put mine into and out of 4hi while in motion all the time, without using neutral. My owners manual states that it has a clutching system to engage the front axle, and it is designed to be used while in motion. It doesn't *hurt* anything to stop before engaging the 4x4, though. If need be I can get the manual out later on and quote directly from it if you require.

Speeds for 4lo shouldn't be more than a few miles per hour. It's mostly to be used when you need extra pulling power (such as towing a boat up a loading ramp or what-not). I've used mine a couple times while going through some nasty snow, but it's rare I feel I need it.

As far as speeds in 4hi, as long as you aren't driving it on hard dry pavement I'd say you're safe at pretty much any normal driving speed. Just remember that even with 4x4, you still have the same amount of brakes as everyone else... Don't drive too fast for road conditions, even in 4x4.

Drive smart, drive safe, and keep that fine ride of yours in one piece.