So I am putting on some flares and need to take off the Cummins C so they fit properly. I got the C off but now there is all the black crap behind the emblem still attached to the truck. Does anyone have any advice on how to get this off?

 

3M "eraser" wheel works excellent. Can be a pricey tool though if you don't have any other uses for it in the future.
Try:
WD-40
3M adhesive remover
NAPA bug and tar remover

 

I bought a pinstripe removal kit form harbor freight that came with a straight air drill and 4 eraser pucks for about $25 and it was a life saver those things work excellent and beat the heck out of the other methods of removing the goo.

 

zippo lighter fuel has always worked for me. After I am done I just apply a little paste wax or take it to the car wash and give it a quick wash and wax.

 

When I removed mine off for flares I used a heat gun to soften the adhesive/emblem up. Pulled them of by hand. The adhesive left on the fender I rolled off by hand and then used a basic foam glass cleaner to clean the area up.