Ok, let me see what I can do. If I'm incorrect, I'll hide the answer.
First, there are 4/21 locations where the munchlax can be. Of course, the determination of where the munchlax tree is is based on a formula using modulus operations and utilizing your trainer I'd and secret trainer id, however, that does not change the amount of trees there are, therefore making the probability still 4/21.
Next, we have to account for the chance of even getting a munchlax. The chances of getting a munchlax are 1%. This is using bulbapedia.
Finally, we need to take into account the actual shiny odds. In diamond and pearl, the shiny odds are 1/8192. Based off of this, let's do the math.
EDIT: I didn't include the math sorry :(
4/21 * 1/100 * 1/8192 = 0.000000232514881 or 0.0000232514881%
The answer is approximately 2.33 * 10 ^ -7, or 1/4300800