I'm guessing under filled based on how fast the pressure drops when the compressor kicks on. What year Cherokee are we talking about? I'm assuming after the switch to R134a if you're using a store-bought can of freon. How much did you put in? I think fill from empty is around 22 ounces.

 

Here is the deal. The pressure will change when the compressor is kicked on. It pressurizes one side and draws vacuum on the other. The low pressure allows the refrigerant to go from liquid on the high side to gas on the low and like water evaporating, gets colder. Then you pass air over the cold side and it gets colder. Pressure dropping on the low side, with the larger hose, is completely normal. All it means is that the compressor is doing what it is supposed to do