probably trans fluid but it could possibly be power steering fluid also....pretty sure Ford uses Mercon V ATF as power steering fluid in all of its late models???
where out of the trans? shifter area? tailshaft seal where the driveshaft goes in? or the case itself? If it were mine I'd repair the leaky seal/gasket before I'd even think of putting some sort of stop leak crap in there...
you fill it thru the fill/check plug on the side of the trans case (driver side I believe) OEM check plug uses the head of a 3/8ths ratchet.....drain plug is on the pass side down low on the tranny case....
to fill it properly you will have to be under the car with the car sitting level whether its on a lift or jack-stands....
your slicks should not make it sit too uneven....you should try to get access to a lift somehow so you can figure out whats leaking...then correct the issue...
if the car has been sitting for a long time since you last drove it there will probably be no visible leak. in my experience most leaks occur when the engine is running/vehicle is moving and during a short period after motor is shut down/vehicle is parked. your best bet is to check and top off the fluid, wipe everything down real good, drive the car for a bit (10-30 miles should do it) then get back under it and see if you can pin-point the source of the leak
Is that avatar pic your car? And slicks in the rear won't make the tranny leak. I asume you've driven up or down a hill at some point in the car, yeah? Take it somewhere if you don't know what to look for & don't put any stop-leak crap in it.
If you have a leak do yourself a big favor. Find it and fix it right. I would NOT recommend a quick fix like a stop-leak product. I'd only use that stuff on a beat on beater. JMO.