Yes, it 'sounds' like they are adequately sealed to use silicone oil, but I have no experience with that particular truck. I'm confident it will be ok, and the worst that can happen is that they will leak, and then you'll know. When using silicone oil, use a nice thick weight for a monster truck, like 5-10k depending how much you want to restrict the diff (higher equals more restricted), and fill them 50-60%