I may be over-simplifying the problem or not understanding it correctly, but if the problem was with the lens, wouldn't that be apparent in the viewfinder as well? I mean, if the focal plane looks right in the viewfinder, it should look right in the picture... right? It sounds like what you're saying is you focus on something, say, 10' away, but the pictures come out looking like you focused on something 10' 6" away. Regardless of DOF (which could just be making the problem easier to see rather than being the problem itself), the focal plane shouldn't be any different between the viewfinder and the photo because it's all coming from the same lens.
Not to insult your intelligence or anything, but could this be as simple as a diopter adjustment to the viewfinder? I'm new to DSLRs so there may be something important that I'm not understanding properly, but sometimes the easiest solutions are the ones most overlooked.