What Mike said.
The auto downshift as you slow down seems to just be a convenience/safety thing. Thus, you can
downshift manually but you must
upshift manually...and you can kick it back to full auto by holding to the right. It seems to also allow you to choose either 1st or 2nd as a starting gear from a dead stop...but I have only played with it a bit...so others may know more.
Regarding OD lockout, Mike is correct, there is no button, but you can downshift to 4th from 5th and it will not go back into 5th unless you manually upshift or kick it back into full auto. Of course, if you slow down quite a bit, it will downshift further...and you will then need to manually upshift back to 4th after you speed up or put in full auto and then kick back to 4th again when you get to your desired speed.
Not exactly as convenient as an OD lockout button, but it will work when you are on a hilly stretch. That said, I have never felt compelled to do so. The 3.6L and 4.1 axle ratios rock.