Why Does My Truck Act Up On Rainy Days?

On my way to work i stop at a light and its raining, crappy day my truck will stall.. It will start chugging but if i rev it at a stop i can keep it from stalling! WHY is it doing this!!! only runs bad on rainy days!!! So my muffler fell off and im kinda wondering if thats why, maybe too much moister is getting inside.. Its a 89 Mazda B2200 great truck! but just this 1 problem. Please help me solve this If it's something i can fix id like to know and ill get RIGHT on that cause it rains a lot where i live. Thank you