Also assuming that this is your '91:
You can remove an entire AC system from a Ranger, at least up to 1994, maybe later, without opening it. It is a LOT of work and requires disassembling the evap housing under the hood, moving a bunch of wiring, removing the radiator, fan, passenger side inner wheel well. You have to be very careful not to damage anything...it's kinda like trying to stuff several angry cats into three bags at the same time.
I did that on my '92 Explorer with my '94 Explorer as a donor and my old '89 Ranger with a different '94 Explorer donor. Both of them had AC to begin with, it was just broken R12 systems that I replaced.
If your truck did NOT come with air, and you are open to buying new parts, I would suggest just buying everything new except the evap/heater housing, compressor bracket and the compressor wiring harness which are junkyard/donor items. Once you have it all installed you will need to add a switch to power the compressor off the donor harness since your heater controls will not have an AC button nor is your dash harness wired for one.