Hello and welcome to JustAnswer!
If you have it on heat can you control the temperature ok on the passenger side?
You could possibly have more than one thing going on here.
With the way the airflow in the heater box is set up one side of the truck gets air that flows over one side of the a/c evaporator, the other side of the truck gets air from the other side. When a/c system charge gets low it will be able to cool one side of the evaporator better than the other, which leaves the right side of the truck's a/c warmer than the left. The first thing I would recommend is having the system evacuated and recharged to the proper level and see if that takes care of it.
If not, then you have a broken door in the heater box. Even if it's a single zone truck there is still a blend door for left and right, they're just linked together. One door can break and the other will still be driven by the actuator, leaving control over temperature on one side but not the other.
To address the other things you were wondering about, there isn't a cabin filter on this truck and there is no vacuum control to worry about.
You can charge the system yourself with gauges and refrigerant cans, but it's not very accurate. The most accurate way to do it is with a recovery machine, so it can be recovered to see how much is in it and then charged to the exact proper level. The price on this at a shop will vary depending on local pricing and labor rates. At my dealer it runs under $100.
The doors on the heater box are accessed by dropping the steering column and rolling the dash back and laying it across the seats. Some doors can be replaced with the heater box in place, some doors require the removal of the heater box to replace.