When I am going about 40-45 mph with rpm lower than about 1700 rpm, the truck seems to shudder a good bit. It goes away when rpm get above 1700 or speed above 50 mph. Any ides what may be causing this?

2001 Dakota Quad Cab 4x4 4.7L V8 Automatic

 

I am dealing with a similiar issue. A couple of possibilities - worn Ujoints, or unbalanced driveshaft. Have you done any work lately that involved the rear end or the driveshaft? Mine started after I replaced the pinion seal.
It could be a missing weight off your driveshaft. Mine seems to do it at low rpms, when you accelerate from a dead stop, there is a small shudder.

 

I get the same kind of thing, but at 1000-1200rpm, above 1200 is smooth. Engine mounts look ok to me. I wonder if it just dosn't like slow running due to the tuning.