While looking through the powers for Savage Melee on a Scrapper I noticed that Shred doesn't have any crit info for it in the power information in-game, while on a stalker it does. Now I can not take credit for this as Talesin discovered this as mentioned on their post here:
Which unfortunately didn't get any replies. Now while talking with about this with Bopper they confirmed it for me and did a screenshot of the power info in the files for Shred showing that there is no crit info for it. So it's impossible for Shred to crit on a Scrapper which I would assume is a bug, since any attacks on a scrapper are meant to be able to do a critical strike since it's their inherent power. Here is said screenshot showing all effects of Shred on a Scrapper proving that there is no critical attack / strike info for it.
For more info on Scrapper Shred I would recommend viewing the following link in FireFox (FireFox comes with an automatic JSON tree reader, making it easy enough to navigate)