I haven't had the intake manifold off yet. The bad glow plug on mine was for cylinder #1, and you can get to that one through the injection tube spider lines. In your case you have no choice.
It seems to me that someone here mentioned a new gasket that you'll need for the intake manifold. And people have said you should clean the insides out while you have it off. There's something about the action of the EGR system sending exhaust carbon through the intake and having it collect there. In the case of the vacuum connection to my EGR valve, I would like to plead the fifth amendment.
That's a good price on Bosch glow plugs - I didn't know Pep Boys had them. I just got new Bosch plugs, and they look slightly different from the glow plug I took out of the car. While the OEM glow plug is the same diameter along its entire heating element length, the new Bosch glow plug "necks down" about halfway along its length to a smaller diameter at the tip. I think this may improve the way it heats - it probably gets hotter now. So if I were buying them in person I'd take a look to make sure they are this newer design. I see that the box says "Duraterm" on it - don't know if that implies the new tip design or not.
In my case with the easy-to-access #1 GP being the only one bad, I chose to replace only that one. I'll bet you could get by for a long time with just replacing the bad ones (I think you have two bad ones) yourself. However, if I were in your situation, I'd go ahead and do them all - considering the effort to remove the intake manifold.
Actually, you might need to remove both the intake manifold and all the injection tubes that run from the pump to the fuel injectors.