Jerry A,
That's pretty standard. I think it has to do with the RPM's and amount of slip to get the truck going in second gear. Did you ever pull the little gravel shield and look inside the bellhousing? Is there any oil or oil mist clinging to the interior surfaces.
Scott mentioned motor mounts...with the parking brake on hard, you can put the truck in first or low and slowly let the clutch out. Do this with the hood open and a friend watching the engine. When the drivetrain starts to lock up, the engine shouldn't move much...if it rocks over "inches" this could be the problem. This is usually evident from inside the cab as the shifter flops with the rocking engine.
Stuart
