This is an interesting question though; DL's shuttle business is doing fine, but if it wasn't MD-88s running it what aircraft could they use? Maybe the 737's but they already don't have any to spare.

I actually think, if again for some reason the MD-88s were not an option, they'd resort to Shuttle America and the Embraer 170's. Take out the 6 seats in first, expand legroom. Maybe the shuttle customer's wouldn't be happy about gate checking every bag, but the aircraft would probably be DL's most reasonable alternative.