Ditto on the capabilities of the 2.8 liter diesel in our 05 Liberty, we routinely camp in the NC mountains with no problem. However, I did a little spec research and discovered that the 04 Sprinter with the 2.7L is significantly heavier than the Liberty (L: 3650 lbs vs S:5160 lbs) and lower torque (L:295 lb-ft vs S:243 lb-ft). If my figures are correct that's almost 40% worse power (torque) to weight ratio
Horsepower figures are almost the same (L:160 vs S:154) but are not as significant for this application, same for the 5000 lb tow rating.
I would still be interested to see if the Sprinter has mechanical issues, but based on the numbers above I wouldn't be too surprised to find that it just doesn't have the necessary power.