What kind of shell? I bought a Leer topper with a missing rear glass back in 2007 and getting a new one-piece rear glass was about $220 at a place called Truck Toys in Longmont. Leer dealers are independent so don't take the first estimate you get. The Leer dealer down the street wanted $400+ to replace my glass (I think he wanted me to buy a new topper from him rather than replace the glass) so I shopped around until I found Truck Toys.
Of course, that was for the SUV-style all-glass rear door. If you have the more conventional aluminum frame with a flat piece of glass almost any auto glass shop should be able to replace the glass for a lot less.
2007 Toyota 4runner SR5, 4.0 V6. FJC lift, FJC steelies, 265/70/17 BFG ATs & custom front bumper!
I'm a million miles before you, I'm a million miles behind
I'll take you straight and narrow, I'll ramble and I'll wind
So curse my broken brimstone, or kiss my bricks of gold
I'm not the reason - I'm just the road
- Aaron Watson, "The Road"