did you pull the extension housing off the trans or are you trying to do it with it still on? When I rebuilt transmissions, I used a bushing cutter which is kind of like a small chisel with a small u-shaped edge on it, but a screwdriver could work. You really need the housing off though. And a bushing installer to help keep the new bushing from collapsing or tightening up too much when you install it.
ETA: If you take the housing off, there is a gasket that you will have to replace. Auto parts stores usually have it. And if you are able to get the new bushing in, before you put the new seal on, fit the driveshaft yoke through the bushing to make sure it isn't too tight. Much easier than finding out when you have a new seal on it that your trying not to damage.