First off, please don't use ATF. It was used as a cost cutting measure. I recommend using Mobil 1 High Mileage Engine oil, 10w-30 or 5w-30 for cold climates. ATF will work but it's not the best choice. It does a fantastic job of keeping the inside of the cases clean, that's for sure.

2nd, if you filled it up enough for it to pour out the reverse light hole (same spot I fill mine), it's too full. What year is yours? The earlier ones (95-98 or so) had a slightly lower oil capacity than the newer ones as the backhalf of the gear case was changed for more oil and with more ribbing to be stronger. The earlier ones only need about a quart.

The proper way to check the fluid is to pull the rubber plug that is behind the driver's side tire. It'll be above and to the right of your axle. Let it drain out until it stops, then put the plug back in.

If it's too full, it may be coming out the vent on the top of the trans, but if that's plugged it'll force it's way out a seal. The fluid expands as you drive due to heat and needs somewhere to go.