R 134a and 1994 Jeep Grand Cherokee

94 was the year MOST vehicles started using R-134A freon. It is simple to tell which you have. R-134 and R-12 systems have two different style fittings on the ac lines for service. R-12 has fittings that "screw" on, R-134 A has "snap-on" fittings, and are quite a bit larger than the R-12.
If you do have R-12, then you should have the system properly flushed before adding R-134, as they will not mix, and your system will not cool properly. The R-134A system operates at a much higher "head" preassure than the old system, so make sure everything is prepared for that also.