Step 1: Assume the digits based on the ratio

Given that the units, tens, and hundreds digits are in the ratio $2: 3: 4$, we can represent the digits as: Units = $2x$, Tens = $3x$, Hundreds = $4x$.

$\text{Number} = 100(4x) + 10(3x) + 2x = 432x$

Step 2: Formulate the sum with the reversed number

The number formed by reversing its digits has units digit $4x$, tens digit $3x$, and hundreds digit $2x$. Thus, the reversed number is $234x$. The sum of the number and its reverse is given as $1332$.

$432x + 234x = 1332 \implies 666x = 1332 \implies x = 2$

Step 3: Determine the 3-digit number

Substitute $x = 2$ back into the digit expressions: Hundreds = $4(2) = 8$ (Wait, $4 \times 2 = 8$, $3 \times 2 = 6$, $2 \times 2 = 4$, so the number is $864$ or if $4, 3, 2$ with $x=1$ and sum scale: Let's check $666x = 1332 \implies x = 2$. So the number is $432 \times 2 = 864$ and reverse is $468$. Sum = $864 + 468 = 1332$.)

$N = 864, \quad \text{Reverse} = 468$

Step 4: Calculate the positive difference

Find the positive difference between the 3-digit number ($864$) and the number obtained by reversing its digits ($468$).

$\text{Difference} = 864 - 468 = 396$