I looked through the diagrams in the 2003 Service Manual in the HVAC section and the two ports you are seeing appear to be the only two service ports on the system.
Both of those ports are located on what is called the evaporator tube.
Both service ports also appear to be different sizes, so a refrigerant refill can set will most likely only fit on one of those ports.
The service port closest to the accumulator (that can-like device near the fire wall) is most likely the service port you are looking for and is most likely the port that will take the refill can line.
Inside that evaporator tube at the location between the two service ports, is a device called an "Expansion (Orifice) Tube".
This is one of the reasons you can feel a difference in temperature between those two areas of the evaporator tube.
It is not just a straight through tube connected at that joint.
This device is located inside the forward most part of the evaporator tube at the junction of the evaporator tube between the two service ports.
You should see high pressure on one side of that device and lower pressure on the other side.
Even farther down the evaporator tube, toward the truck frame and closer to the front of the truck, is the A/C refrigerant filter.
It is a device located between two sections of the evaporator tube.
Not that any of these devices might be a problem, but anytime you have a connection joint in a refrigerant line, that is a possible location of a refrigerant leak.
As you probably already know, a healthy A/C system can easily hold a good charge for the life of a vehicle.
My 1993 Ford Explorer still has it's original Freon charge from the factory, some twenty three years ago.
If your A/C system is really low on refrigerant, you have a leak somewhere that will never go away and needs to be found and fixed.
Adding an odd can of refrigerant now and again will never allow you to achieve the correct level and get the proper performance of the system.
Getting it fixed correctly will end this hassle and will protect your system from more costly repairs down the line.
But that's just how I prefer to handle these types of things.
Hope you find a solution that works for you soon.