1997 dodge ram 3500 starts but can't bring rpms up.

That sounds like an exhaust problem, if your cat. converter is bad your vehicle will start but have loss of power and you will struggle to get to and above 2,000 to if your lucky 4,000 rpms.
A good way to tell is if you run your vehicle for an extended period of time (specially driving), you can get under the vehicle and you might actually see the exhaust system close to the converter is red hot.
Your truck can run without it but it will fail any emissions test you take, it is located between your headers (on the engine block) and exhaust piping under the vehicle.on.
Hope this helps and the cat. converter usually has a manufacturer 5 to 10 year warranty that will cover replacement, if not it should run around $500.00.