The 91-92 and some 93-94 Explorers came with the smaller FX-15 A/C compressor for R-12 systems, since R12 doesn't need very high pressures. The later 93-94 Explorers came with the larger FS10 compressor, which was more robust and better for the higher pressures of R-134a.
You should be able to use an FS10 compressor from just about any 93 to 2001 Explorer with the 4.0L OHV engine and it should be a direct swap. The FS10 compressor will of course be larger, but the pulley should line up with the serpentine belt.
If the A/C hoses were never replaced, you can buy the aftermarket hoses for the 91-93 Explorer and they will be the newer "barrier" type hoses designed for R-134a. R-134a molecules are smaller than R-12 molecules, so using R-12 hoses allows the refrigerant to escape through the hose walls.
You should also add a new accumulator/receiver drier, with either a variable orifice tube or the R-134a specific tube, and an R-134a pressure switch. These items are relatively inexpensive but make a big difference in terms of having a whole setup for R-134a compared to just sticking R-134a into a system that is made for R-12.
