For a starter you can do some rule of thumb estimates.
The 2000 dodge Ram truck has the following aero numbers:
http://www.pickuptrucks.com/html/ram_specs.htmlTwo-Wheel Drive Regular Cab Pickup
Model 1500 1500 2500 3500
Frontal Area 34.8 ft2 34.5 ft2 34.8 ft2 39.8 ft2
Drag Coefficient 0.44 0.46 0.44 0.45
That will give you a ball park number to start with.
If you cannot find published numbers you are then limited to doing tests to figure out a representative number of your own.
Is the truck drivable now? If so you can do coast down tests, or if you have access to a wide open stretch of road you can also estimate the CD based on the top speed at a much lower power level.
http://www.eng-tips.com/viewthread.cfm?qid=10965The basic concept of the coast down test is very simple. Inflate your tires to maximum rated pressure to minimize rolling resistance, then you accelerate the car up to a target speed above your test range, kick it out of gear to eliminate transmission power losses in the gears churning oil, and measure the time it takes to change from one speed to a lower speed. For example accelerate to 70 mph then go to neutral and with the absolute minimum steering input let the car coast down from 70 to say 60 mph on level ground. Use a stop watch to measure the time it takes to drop from 60 mph to 50 mph. Your average speed over that interval is 65 mph. The just figure out the force it took to slow the car 10 mph in the time interval. If possible do the tests multiple times in opposite directions on the same segment of road way to cancel out local wind effects and slight grades on the road way.
http://www.stockcarscience.com/scienceTopics/scs_Aerodynamics_Drag.phpSince older trucks had higher aero drag than newer trucks I would bet your CD is most likely between .45 and .50
Larry