I'm having the same issue with my 2006. You can hear the lock close around the striker when you close the door, but it seems to immediately open again and the door isn't latched. You can lift it with a finger, like others have mentioned. Door ajar light on dash comes on as well.
With the rear hatch open I can push on the latch with a screwdriver and the motor engages to close the latch. It stays in that position, but it's not locked in that position and I can easily pull it open again with a screwdriver. I've sprayed the crap out of it with lube and cleaned it up with a wire brush. I also tried the fix of closing the door and hitting the lock button the key fob immediately, but still nothing.
So it looks to me like the latch is closing fine, but it's getting a faulty signal to open again or not to latch. The push button on the rear hatch to open the door doesn't activate the latch mechanism at all, which leads me to believe it's actually the switch assembly that's stuck in the "open" position, which is a $40 part vs the lock mechanism which is more like $300. Is that possible?

Anyone have any experience with just replacing the switch assembly vs the entire lock actuator?
Thanks!!

UPDATE- 6 months later...

I had the part number and was all ready to order a new actuator ($300+) when suddenly my lock started working again! I haven't had an issue with it again since.. and it's been about 6 months...knock on wood. Now I wonder if I have electrical gremlins. I'm always amazed at how this truck just keeps going and going. I'm at 398,000K and it's still my daily driver. I'll weep when I finally have to say goodbye.