Re: 4x4 won't engage
What do you drive? Is it electronic or manual? Have you made any modifications to the suspension/tires, etc.?
Sometimes the electrics won't go into or out of 4wd when there is a bind on the drivetrain. If this is the case, get off of the concrete and drive with the wheel fully turned to one direction or the other (driving in a circle). After a couple revolutions, try to engage the 4wd. If this doesn't work, go the other way and try again. Also try reverse if needed. If this doesn't work, bind is robably not the problem...start looking at the electrical parts(switches, relays, etc.). Oh yeah, first make sure that the fuse is not blown.
If it's manual, are the hubs locking? Is the linkage broken, etc...
Write back if this doesn't answer your question.