The graph below traces average wholesale selling price of Class 4 and 6 Conventionals from January, 2008 to May, 2013.

Both segments recovered mildly in early 2010 along with the rest of the commercial vehicle market. Class 6's came on-line more notably in 2011. Mileage and age mix during that period was similar to earlier periods, so the increase in pricing at that time appears to be a natural increase in demand. Both segments saw a notable bump in early 2012. In the case of Class 4, a large number of newer trucks cycled through the auctions during that period - most likely a fleet trade package. Pricing returned to previous levels after those lower-mileage trucks were "cleaned out" of the market. In the case of Class 6, that segment was likely enjoying a favorable supply/demand relationship until a large number of trucks was introduced to the market - possibly a large package of trade-ins from a rental company. This sudden influx of trucks of average age and mileage brought pricing down substantially. Fortunately, supply appears to have reached a more favorable level in recent months, and pricing has increased as a result. 
We’ll need to see higher pricing in Class 4 and more stable pricing in Class 6 before considering the recovery in those segments to be clear and substantial. At this point, it’s safe to say that conventionals in general are clearly performing better than during the worst of the recession. Class 6 should continue to improve, with a chance that large trade-in packages could again negatively impact supply. Class 4 appears to be in an equilibrium of sorts – demand will need to increase before we see notably higher pricing.