If fuel pressure is low and you have already replaced the fuel pump, then the problem is usually the fuel pressure regulator. The pressure regulator is in same assembly with fuel filter. The assembly is located to the fuel tank, bolted to the truck's frame. The pressure is tested connecting a gauge like this : Set Sealey VSE952
to the port on the fuel rail. The pressure on Grand Cherokee must be around 49 psi plus or minus 5 psi. If engine runs but pressure is below 44.2 psi , check for a blockage in fuel supply line between fuel rail and fuel pump module (e.g.kinked line). If the line is not blocked, replace fuel pressure regulator and filter. Test pressure with and without fuel cap on.
Another test to be done is reading pressure, then shutting the engine off with the gauge connected. If pressure goes down too quick, then the regulator is defective. After 5 minutes the pressure should not go below 24 psi. If it does replace the regulator.
If regulator is OK check for leaks at the injectors that can cause fuel pressure drops.To check for injectors leakage clamp off the rubber hose of the pressure gauge tool between fuel rail
and the port. If pressure staysr above
24 psi and before was dropping, then a fuel injector or the fuel rail is leaking.
Below a diagram representing the fuel rail and test port. The engines from same years like your six cylinder have the port and rail in the same position.