When I make buckeyes using this recipe from Smitten Kitchen, I tend to reduce the amount of butter a little, and the sugar a lot. Then I increase the graham crackers, and add some low-sugar puffed rice (Rice Krispies-like cereal) for texture. You might be able to work with some "filler" ingredients like that to make up for the lost fat from the butter. The cream cheese in this recipe probably helps to counter any reduction in butter so that might be worth experimenting with in various amounts; you could use Neufchâtel cheese instead of cream cheese to reduce the fat there.
Another great option is homemade peanut butter. Your basic creamy Skippy-brand peanut butter has 17g of fat in 2tbsp. Creamy Jif has 16g in 2tbsp. Depending on what recipe you use, you can reduce that quite a bit - this recipe has about 11g of fat in 2tbsp. That's a huge difference when you scale it up to 2 lbs!
Obviously with all these substitutions they won't taste exactly the same as you're used to, but for what it's worth, I made the buckeyes exactly as the recipe said the first time; the second time I modified it as I said in the first paragraph (using store-bought peanut butter) and I preferred the latter much, much more. The original version was just too sweet...and the original version is a modification of another recipe that used double the confectioner's sugar!
Since this treat you're making is likely similar to the buckeyes (in that you don't need to bake at any point), I would highly recommend cutting the ingredients down to very small amounts, and try making a few different versions and having a taste test with some variations.
As an aside, one tip I learned through practice: if you're dipping peanut butter treats (instead of pouring the melted chocolate onto them), and are having trouble with them being too crumbly when you reduce the butter and thus they are falling apart into the chocolate, you can freeze the peanut butter balls/treats for a while to make them firmer - this makes them much easier to dip.