Can I back probe the ecu to check ECT & IAT sensor resistance?

 

Why don't you just check it at the sensor like most people do?

 

Because its in a b**ch of a location

 

If you're testing resistance to see if a sensor is bad, then no. It has to be done at the plug, because the wires add resistance, and more often than not the wires are the problem. But you can always remove the sensor and test it, since a thermistor doesn't need power for ohms readings. 99% sure your sensor is fine.
But really you should be measuring voltage anyway. Which ideally should be done through datalogging, but voltage at the ECU plugs is the most accurate analog reading. If voltage checks out, your ECU is bad. If it doesn't, it's probably a bad ground connection at the thermostat.