Is there a diagram available that will clear up the proper line to connect to? I really know which one; it is the large or low pressure line. On my truck it is actually at the manifold near the compressor, however the fitting is to big for the charge line to attach to. The high pressure line (the smaller line) has the connection that fits the charger connection. I suspect the fittings may have been swapped to eliminate the average person from filling the system. I am assuming the based on a comparison that I have made to other vehicles.
a/c charge ports on 1996 Dodge Ram 1500
That does sound odd, and it does sound like they are switched. I would take it to a Dodge Dealership and see if you can get a tech to take a peek at it, they would be familiar with the proper positioning of the valves. Whatever you do, don't attach the can to the high pressure line while the engine is running.