Turned on 4X4 going 60MPH!

I was going about 40 MPH (not 60mph) on my recently purchased 94 4Runner 4x4 V6 auto. While driving I put the gear from H2 to H4 (4x4) and I heard loud clunking under the truck. First time I tried the 4x4 and all i know is now when I put it in 4x4, there is clunking under the body (metal on metal) and the tires screach when trying to go in reverse. The truck pulls HARD to the left and it's almost imposible to drive. 2wd mode drives perfect. Any ideas what may Have broken/disconnected or what I need to check? Thanks in advance~

do you have auto or manual hubs?

Your front differential is probably damaged in some way. Since the truck drives fine in 2WD, the axles are fine from the wheel to the differential. Since you can switch into 4WD, your transfer case and front propellor shaft are OK. The only parts left are the ADD actuator system and the differential.

Not sure what kind of hubs I have. How do I check that?
Is teh ADD actuator replacement a DIY and I don't think it would lock up the wheels (rear) the way it did and make them screach. As far as the diferential is there a way to test to see if it is damaged? Appreciate the responses and I may go to my mechanic monday. I reached 35mph in the current condition and I didn't try to go faster. a bit of clunking coming from under.

Edit:: BTW are there good sites w/ pictures of the under carraige (and part labeling)? Been searching and no luck.

If you have a V6, you have ADD as standard equipment. If you have to get out and lock your hubs, you have manual hubs (someone may have installed manual hubs). If you don't have manual hubs, you have ADD. I don't think it is your ADD actuator either, but it is possible. Yes, you can replace the ADD actuator. It is on top rear of the front differential housing. I think the fact that you have big trouble in 4WD and no trouble in 2WD shows your ADD actuator is working.