Ah yes, the 4L60E pukefest! I have 3 huge stains on the parking lot because of this- landlords just love me right now. Mine, and the others that have had the same spot in the same place as yours (and mine) have been caused by minor overfill that pukes out of the dipstick tube seal, onto the crossmember, down to the exhaust pipe and all over the ground. So much fun! Start by wiping everything down with rags for the heavy stuff, then brake clean to get the rest and leave a dry surface. Warm the truck up with a 20 minute or so drive, park on a LEVEL surface (OH is this important!) and check your fluid level, you're probably a bit high.
I had the misfortune of bending the dipstick stop bracket on the transmission case while working down there and that was giving me false readings. If I let it sit, go out and start it without driving it (haven't started the truck in a week or two, think I'll run it for a few minutes and let it warm up). Within hours, here we go again, a big red puddle resembling Lake Michigan is running down the parking lot. Again. Hope this helps