Where in the midwest is it $2.97? Its $3.59 in St. louis. MO
Anyway, the oil companies are driving diesel fuel prices up because they are fighting the automakers trends to develope more diesel powered vehicles in the US. Diesel is more efficient than gas and the consumption will ultimately be less if the US converts to diesel like the rest of the world. The oil companies obviously do not want more efficient vehicles in the US market.
Ultimately I don't think it has hurt the value of diesel trucks. Their superior to gas fuel economy still makes them hold their value really well. In the long run though as the US makes diesel fuel more and more environmentally friendly our older diesel trucks may eventually not be able to run these new fuels without alterations, which are likely to be very expensive.