Help Silverado brake and power steering fail

[rtavi]

I have a 2003 Silverado 2500hd diesel with Allison trans. Long bed crew cab. Truck has32,ooo miles with no other problems other than scheduled oil changes and trans fluid and filter at 30,000miles
Two weeks ago I went out to start it on Monday morning after it sat still from Friday afternoon. When I started it with trans in Park and put it in reverse to back out of the driveway the steering froze and the brakes were impossible to press down. It felt like the hydraulics were gone on both. A tow truck came and the driver finally manhandled it straight enough to be towed to the dealer. At the dealer that afternoon the mechanic started it and everything worked fine so they told me to come get it. This weekend the same thing-it sat still from Friday started on Monday morning and brakes and power steering wouldn't work. I turned it off and tried again -no luck. Then I had an idea. I put it in neutral and started it-everything worked OK. I took it back to Chevy dealer and told them what happened. When I returned the next day they said they couldn't find anything wrong and couldn't fix it if nothing was wrong. Does anyone have any Idea of what's going on?