unfortunately it sounds like you have 2 different problems, the over heat shouldn't have anything to do with shifting, sounds like there might be a tranny issue, possibly related to the torque convertor, hopefully it will be something simple like the shift linkage coming loose, but i doubt it, the coolent had to have gone somewhere, check the engine oil level(dip stick) and make sure it isn't full of water, that small of a leak wouldn't have drained you coolent dry, did you pull the rad. cap off or are you talking about the overflow tank?