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I have seen the problem you describe. There are ground wires on the back of both cyl heads. They are the grounds for the 4wd system. If they are loose or the wires are damaged it will do exactly what you have described. Check them first before changing any parts. Also, if the system has seen a malfunction, sometimes you have to let the system relearn. Pull the 4wd fuse for 3 min. Then after reinstalling the fuse, turn the key on and off 5 times for about 5 seconds per time for the 4wd computer to relearn. Good luck. Greg