I was under the impression that decoction helped negate the effects of under-modified malts, but wouldn't cause any problems with fully-modified ones. I'd be inclined to think that you'll have no problem decocting full-modified 2 Row.
You had three pounds of crystal malt in your grain bill? Well, that's going to contribute a ton of unfermentable sugars to your beer. That's one reason your FG is so high, and the other reason (like Hammy71 said) is that you mashed at too high of a temperature.
Any particular reason you had 3...
It would be MUCH more beneficial if you could describe the off-flavor better than just "an aftertaste". Does it taste like canned vegetables? Or like buttered popcorn? Is it sour? There's a ton of possibilities, and nobody can really help pinpoint your problem until you give more information.
A bit of pale chocolate malt should give you a nice reddish-copper hue. I just brewed a West Coast Red Ale today that used 2 oz of pale chocolate along with a quarter pound of C120 & a half pound of C60 and the color going into primary was a nice deep copper.