If you think it may be low on freon and not closing the pressure
switch you can jumper the pressure switch for just a few seconds to
verify your compressor clutch is engaging.  Just don't run it like
that long because if you are low on freon you may be low on oil
compressor oil as well and that would cause significant damage to the
compressor.  You can do a lot of the preliminary checks yourself.
Check most of the fittings and the compressor for wet oily areas, if
you find any you will probably need to replace o-rings.  If the
compressor is oily and you can verify it isn't engine or power
steering pump oil then it will need to be replaced or rebuilt. I'm not
positive if the 22R compressor is the same as my 3VZ 91 Truck but if
it is you can get them resealed for about $65.00.  The was the price
two years ago from AAPAK in Phoenix.
 The 90 4Runners has a recall on the expansion valve under the dash,
you should check with the dealer and have them run your VIN.  If that
hasn't been done you may be able to get them to change it out and take
care of testing the system at the same time.  If the expansion valve
is the culprit the entire job may be free.
 Another note, if the compressor needs to be resealed or you replace
it, fill it with ESTER oil.  Ester oil is compatible with all
refrigerants just in case you ever decide to do a conversion.  I run
Freeze12 in my truck, its $8.00 a 12oz can.  Doesn't work quite as
well as R12 but I still get 38 degrees or so out of the vents here in
Phoenix.  R12 is still available for around $20 a 12oz can according
to one of the suppliers here in Phoenix.