Well I went to see my mom over the weekend and parked my truck at an incline for 3 days. Then I got in it and started driving home, and about 75 miles into a 300 mile trip, the O/D light started blinking on the dash. Not thinking much of it, I kept driving. When I stopped for a rest about 160 miles into the trip, the truck was slightly smoking under the hood. I looked under it and the transmission fluid was leaking all over the ground. ( about 1/2 a quart. So I let the truck rest. Got in it about 20 minutes later, and the over drive won't work and the truck is shifting hard now. No more leaks but it is not shifting right.

Can anyone tell me why the fluid boiled out and now the O/D isn't working? And why is the truck shifting so hard?

In West Texas, it normally happens in the summer heat, while hauling a load. It just boils out. I would drain and clean the pan, change the filter, and refill with Mobil 1 Multi-Vehicle ATF. The transmission should then run 25 to 50 degrees F cooler and perform much better. Depending upon where you live, I would install a supplemental transmission cooler. Change fluid and filter every 30,000 miles.