02 Tacoma slipping into 4WD while driving

I was driving my 2002 Tacoma down the interstate this afternoon and while going about 70 it started making this weird grinding type of sound and suddenly shifted into 4WD drive making a loud thump. I pulled off the road switched back to 2WD and then started driving again and about 30 minutes later it did the same thing. That being said while i was driving i had on the cruise control and was not shifting or anything. Anyone know what could be causing this?

 

Manual or automatic transfer case ?
Manual or auto transmission ?

 

yep, def. need more info

 

It's a Manual

 

Manual hubs? Manual transfer?

 

^I've never heard of a rig popping out of 2wd and into 4wd before. But, I have heard of rigs popping out of 4-low into neutral. Maybe it's the same cause... a worn shift spring:

http://www.marlincrawler.com/hardware-tools/hardware/heavy-duty-shift-spring

 

Another one of those posts!!!!....