Checking whether two voltages are similar
You are actually talking about performing three functions: subtraction (or difference), absolute value, and comparison with a fixed value. For determining the difference between the two input values you need a differential amplifier. There are a number of such circuits that use an operational amplifier, and the proper choice may depend on the source impedance of V1 and V2. If these inputs have a low impedance you may be able to use just one op amp to compute the difference, and in the worst case scenario you would need three op amps. The absolute value can be computed using one op amp and a couple of diodes configured as a precision rectifier. Finally, you need a standard comparator where the trip voltage is set by a ratio of fixed resistors or a potentiometer.

I don't think the LM324 is a particularly low power op amp, and it's not clear what you even mean by the phrase "low power", so I'll leave that issue for another question.