Which engine? 4.0L OHV V6, 4.0L SOHC V6, or 5.0L V8?
I'd hook up an OBDII scan tool and look for codes set. If it can show live data like fuel trims, so much the better.
Does it run well above idle? If so then I'd try cleaning the IAC valve first and if the A/C is on, turn that off to see if it makes a difference.
Could just be a neglected maintenance item like old spark plugs, bad spark plugs wires, dirty IAC valve, dirty MAF sensor, or something more significant like a vac leak, for example the intake manifold gasket, or something else.
Any further info you can provide might help to narrow it down. Is it a regular stumble or comes and goes and if it comes and goes, randomly or some pattern?
