F150online Forums - Aod Hard Downshift

I Hope you guys can help me on this one. I have a 1991 ford F150 with a 5.0 and an AOD. MY question is, when i either force the transmission to down shift either with the gas or with the shifter, out of OD I get this very LOUD bang that jarrs the whole truck. The faster i am going the worse it is. At first is though my truck had some kind of shift kit in it, but it only does it when downshifting from OD to 3rd. I checked the tranny fluid and its a little dark and smells a little burnt, so that worries me that i have a tranny problem, but it only happens on the downshift. I checked the U joints and they are tight, and i also checked the transmission mount. the trans mount had about 2 inches of up down play in it. Now is this enough play to make this Banging noise, or does it sound like its a problem inside the tranny or could it be something else? I have owned 6 cars and trucks with this AOD and have never run into this problem. I dread having to run in OD right now cause of this problem, cuase it jarrs both me and the truck, and im worried it might shake the old girl apart. I hope you guys can help me with this. thanks in advance