I've just had the flashing glow plug warning light on my 2.4 JTDm. When the glow plugs fail they go high resistance. Not just slightly, but completely (ie open circuit). I checked mine - they're between 1 and 2 ohms - the resistance of your multimeter test probes and dirtiness of the metal contact will make the biggest difference to the readings - I wouldn't get picky over half an Ohm. The dead one was open circuit! The Bosch glow plugs on mine (Bosch Part: 0 250 203 001) are also capable of a thing called "afterglow" where after the initial starting, the plugs are active to reduce diesel clatter and improve emissions for a short while after the engine starts. I wouldn't recommend cheap ones and would replace like with like. I used "The Green Spark Plug Company" who seem very good. Oh yes, don't bother replacing them as a set - they are not spark plugs - you wouldn't replace all the bulbs in your chandalier if one went, would you?
If *all* your BOSCH glow plugs have resisitance of a couple of Ohms I'd put my money on the glow plug control unit.
Best of luck with it!