Hi I'm not a mechanic but I would probably check the fuses and relays first. There is a fuse box on the passenger side and under the hood. I would search the forums/google find out what fuses and/or relays control the power locks and rule out a bad fuse or relay. If you do find out there is a relay under the hood that controls the power locks you can swap with another in the fuse box that is the same type and rating.
Other than that I would say it's the BCM at fault. A trip to the local junk yard should suffice. Although I believe you will have to goto the dealer to reprogram the new BCM.
I've owned 3 of these vans from a 1998 Pontiac Transport, a 2002 Oldsmobile Silhouette and now a 2008 Chevy Uplander all basically the same drive train with slight upgrades over the years, body designs..
My 2002 Silhouette had the strut bearings going out and getting worse by the day, the steering would bind when turning and you have to turn it back to center it would not return on it's own. Replacing the two front struts fixed that problem.
Now on my 2008 Uplander my power steering pump is on the way out and I plan on replacing it and the serpentine belt also later this spring. Every now and then when the van is at low rpm (ie.. idle) and I'm moving through my parking garage I loose the steering momentarily and the belt squeals or when I hit a big puddle driving the belt also squeals. So for about 200 on rockauto parts I can get a Delco OEM replacement pump and belt and borrow the pulley puller from local parts store and a Saturday afternoon should do the trick..
Hope this help's.
Cheer's..