The first thing that jumped into mind for me was an article regarding chain suck, that mentioned problems with putting a new chain on a heavily worn cassette. The problem you describe isn't chain suck though, but I could see where it's possible a new chain could cause issues.
Don't read the entire thing, just see the pictures that shows 'worn teeth, new chain' to get an idea of what the worn teeth look like. To me your rear cassette teeth look OK, but I'm not an expert on it.http://fagan.co.za/Bikes/Csuck/
Like Greenfast said, start troubleshooting by determining if it happens in every gear, or just in the large front gear. Go to a parking lot, shift through all your gears and crank on it. Make note of which gear combinations cause the problem.
I had Scheels do this tune up right before I did a cyclocross in DesMoines a few weekends ago. And as I was killing time waiting for the race I noticed a lot of grease and gunk on the sprockets and de-railer. That was my first hint they may not have actually fixed anything...So I'm a bit worried where my $120 went.
I would call Scheels and ask them exactly what they did, just so you get a better idea. I'm guessing that gunk is probably from the new chain, they are usually coated in the stickiest gummy grease I've ever seen. I like to get that junk off, clean them right away and re-lube them, so they don't collect so much dirt.
And about the chain length.. new chains are extremely long, is it possible Scheels didn't adjust the chain length when they replaced it? Did you put the bash guard on yourself, and shorten the chain when you put it on? Without a large front ring, you can usually take quite a bit off.
I could see the extra slack the derailleur is taking up is causing it to sit differently & act differently. Another thing you could mess with is the "B adjust screw" on the derailleur, just to see if it has any effect. http://www.parktool.com/blog/repair-hel ... derailleur
Use the process of elimination, closely observe the chain while pedaling slowly & see if you notice anything out of line, any skipping, etc & post back what you find out. If you need a diff. front chain ring to try to eliminate that as a test I'm sure someone on here would have a middle ring you could try. I have a large one if you want to try that.