It really sounds like a vac. line issue to me, the only other thing I can think of is if the throttle cable got moved around some (but really shouldn't have changing the plugs).
Some of the 4.7's have unused vac. ports that are capped off, it's quite easy to knock the rubber plugs off, they are on the drivers side of the intake, just behind the throttle body (where the other vac. lines connect).
If they are OK, I'd check the PCV hose, it connects to the passenger side rear of the intake, I've knocked these off a few times changing plugs.
If that's OK, then I'd pinch the vac. line off one at a time where they come out of the manifold, it's possible that one knocked loose on the other end, if that makes sense to you. If the RPMS drop when you pinch the line off, follow it and see where it may have come loose.
Also check to make sure that the brake booster valve didn't get partially pulled out of the brake booster, that is also easy to do.
So basically , double check it for any type of vac. leak that may have happened while doing the spark plugs.
Let me know how you make out, and we can take it from there. Please don't click accept unless this answer is helpfull to you.