You mentioned I-94, which passes though Illinois, so I thought I might point out this little tid-bit of information:
Illinois is somewhat well-known for being a hard-ass state as far as laws and regulations go (especially with regard to driving privileges.) However, this one little gem of a press release from Illinois state senator Dan Rutherford almost makes up complete for it:
"Springfield - For two years the state's 'left lane law' has sought to increase safety on Illinois' highways and interstates by making it a moving violation for a driver to remain in the left lane of a multi-state highway and detain the traffic flow, said State Senator Dan Rutherford (R-Pontiac)." Found here: http://www.danrutherford.com/leftlanelaw.asp
You argument about going the speed limit is completely irrelevant. Speed has nothing to do with obstructing traffic in the left lane. If you are obstructing traffic, move over. Period. Failure to do so is in violation of Illinois (and some others) state law.
Going through the press-release, they even make an informal mention to the exact situation you bring up:
"...slow drivers in the left lane aren't just an irritation, but can be a serious driving hazard leading to road rage, tailgating, increased congestion, and of course accidents, on roads where people are driving at high speeds."
If someone is coming up on you, you really should just man up, move over, and be safe. Even move back if you want (given there isn't anyone else behind you, see: bullet points of the press release.)
Here is a link to the actual Illinois Statute that states all this information. I direct your attention to section 701: DRIVING ON RIGHT SIDE OF ROADWAY; OVERTAKING AND PASSING, ETC. It outlines everything in fairly plain language.