You really don't have to completely drop the tank, in order to do it. I'll try a quick step-by-step for ya:
1. Put car on jackstands
2. Position floor jack underneath tank with block of wood to spread the load
3. Unbolt filler neck from inside fuel door and underneath near frame rail
4. Lower tank enough to be able to manipulate filler tube out of tank
5. Pull old seal and coat new seal with motor oil
6. Insert new seal and coat filler neck with motor oil.
7. Reverse steps 4-1 and you're done.