Peanut Butter Muffins - Your advice needed

I made these muffins the other day using a recipe I got from Nic of Baking Bites. Her recipe called for all-purpose flour; I substituted half of it with white whole wheat flour. I also used Splenda Brown Sugar Blend, following the directions for its use by using half of the amount in the original recipe for regular brown sugar.

They were absolutely wonderful out of the oven - but within a day, they were very dry. How do I make them moist when I substitute like that?

PB & J Muffins

from Nic of Baking Bites

2 1/4 cups all purpose flour
2 tsp baking powder
1/2 tsp salt
2/3 cup brown sugar
6 tbsp butter, melted and cooled
1/2 cup crunchy peanut butter
2 large eggs
1 cup milk
approx 1/3 cup jam (any kind. I like raspberry)

Preheat oven to 375F and line a muffin tin with paper liners.
In a large bowl, whisk together flour, baking powder, salt and brown sugar.
In a medium bowl, whisk together the melted butter, peanut butter, eggs and milk until smooth. Pour into flour mixture and stir until just combined.
Fill each cup of the prepared muffin tin about halfway up with batter. Top each with a heaping teaspoon of jam and cover with remaining batter. Each muffin cup (of a standard 12-cup muffin tin) should be filled to the top.
Bake for 17-20 minutes, until a tester comes out clean and the top of the muffin springs back when lightly pressed.
Cool on a wire rack.

Makes 12.
(I made 36 mini-muffins and 6 large ones)