Sounds like my truck in the winter mornings! You were wise to try it without the hydro belt. We now know it's not the drive system. Next thing I would check would be the clutch. Sometimes the bearings freeze up and lock the clutch up and it makes you think it's the engine is frozen. Best bet it to remove the clutch. In this case just removing the belt won't tell you anything. Sadly you are going to hope its the clutch..