I have read and experienced it once first hand, where a seal puller/pick just wont budge that seal.
What I did and have also read about, is to put a flat blade screwdriver at the edge of the O ring seal, then tap the screw driver in towards the seal opening. This will distort the O ring, which has a metal band around it and then it's easy to pull it out with some long nose pliers.
BE CAREFUL not to scratch or damage the valve body when you do this.
I'd also especially if you this is a new to you truck with no maintenance history, do a full fluid change as well as new filters.
Use the "disconect the transmission cooler return line" and pump out the tranny method for this. There are several write up's on here walking you through this. That way, you get all the old ATF+4 out and new in. If you do a pan drop only, it leaves 8-10 liters/quarts I believe (someone will chime in here with the correct amount) in the torque converter. Either way, it's a lot of old fluid circulating around. Contrary to popular belief, replacing all the fluid in this way is good for your transmission. Even if it has high miles on it without a fluid change. Just don't take it to a shop and do a power flush. This does flush through tiny bits of dirt and debris through the tranny, causing potential problems to an old high mileage transmission with poor maintenance.
Hope this helps?
Al.