Not knowing exactly what system you have it is hard to say. You might consider contacting a truck repair place and ask for an estimate which might provide enough detail to start doing more research.
I change my refrigerant in my 89 GMC Suburban. I changed the air dryer and all the O-rings, the orifice and blew all the lines out. Added new oil, pulled a vacuum and added the refrigerant.
2002 Monaco Windsor 38 PKD
2005 Jeep Grand Cherokee w/5.7 Hemi