One company that seems to compete with Enterprise's truck rentals is U-Haul. There is also the Penske Truck Rental company, which also competes with Enterprise.
I believe it is Enterprise Rent-a-Car. They are the largest annual single buyer of new cars in the United States, and the fleet is approximately 750,000 vehicles.
Uhaul is probably the cheapest, but be careful when renting from them since many of their local offices are not reliable (check their BBB record to see why).Budget or Penske would be a more reliable...