Did I blow a turbo?

I run a 2007 379 short hood with a C15 625 twin. Today I was loaded with 79.5k, climbed a few hills and when I started down a grade I let off the throttle and let the engine brake do its thing. I was in 18th gear around 70 MPH @ 1,500 rpms, then I heard what sounded like a tire blow out that was slapping the fender, then I looked in my mirror and saw literally nothing but grey/white smoke. I immediately threw it in neutral, got off to the shoulder and shut it off. I opened the hood and found no fluids leaking or any soot anywhere on the engine, just smoke still hovering out of the stacks. All my gagues read normal when I was driving, manifold PSI was around 55 while I was climbing the hills, which is where it always has been since I've been driving it. The truck has about 899k on it and the engine was rebuilt at 310k. I asked the mechanic at the shop what his thoughts were and he said he'd get to it when he could...any thoughts?

 

Awesome. What are the chances that it damaged the engine do you think?

 

It's a company truck, they're pretty good about checking over everything it's just that they take their time doing it. Now I'm stuck with a Detroit. I looked at all the boots around the turbos but I didn't notice any that were cracked or damaged, then again I'm not much of a mechanic. I'm guessing both will need replaced if the top turbo blew out? Sending broken fins into the bottom one? Or wouldn't it matter which one