Have a 02 Chevy, DMax/Ally. I am new to these trucks and can't figure out how to lock out overdrive (not that I've needed to yet). I understand tow /haul function and what it does. Owner's manual says nothing about OD lockout. Read somewhere that if you hold down the tow haul button for 3-5 sec, it'll lockout OD. I've tried this at all different speeds from dead stop up to about 50 mph (much over that and you're out of 4th anyway) and can't get OD to lock out.
Is this normal, or is the trans smart enough to not upshift if pulling a grade w/ a load? Any help appreciated.

You have to add a switch and wire hooked up to the PCM.
http://www.kennedydiesel.com/docs/Allison%20OD%20lockout%20final.htm

Thanks for the info MaxPower. That definately explains why it doesn't work. I guess if I get into some heavy towing I'll look into that.