marksman

I have a stock 05 2500 ctd auto. I pull a 3-4k lbs trailer and when I go up this one large 1.5 mile mountain it sems to defuel or not give as much power. Only in tow/haul mode though. I know the weight of the utility trailer is nothing but the grade is steep enough that all semi's are under 35mph with there flashers on. Does this seem normal? I purchased this truck cause the 1500 hemi seemed like on this hill. Thanks