it could be a number of things.
per-factory shop manual,
issue: Cool air comes out only at high speed
Condenser clogged
Drive belt slipping
Compressor faulty
Insufficient or too much refrigerant
Air in system
==========
trouble shooting info on your a/c can be had here, covers 90-95
http://www.ncttora.com/fsm/1993/airc.../7troubles.pdf
the entire FREE factory shop manual for your truck can be had here,
http://www.ncttora.com/fsm/1993/1993.zip
or pick your poison from here,
parent pg.
http://www.ncttora.com/fsm/
owner 93 toy pu 4x4