Since you didn't mention anything about unusual running characteristics, or a stinky exhaust, my guesses are
1) the timing is very advanced, or
2) that the throttle blades are simply not parking where they should be parking. Which is up the transfer slots, exposing them to be about square.
a) As to the why of these, that you will have to figure out.
b) As to timing, the flyweights could be stuck, or the springs broken,
c) As to the throttles;It could be as simple as she be stuck on the fast-idle cam, or the curb-idle is cranked in too far. It could be a failed or maladjusted anti-diesel solenoid. Or the throttles are hung open by a base gasket, or the KD linkage. Or a failed PCV system.
The reason I suggest the above, is that, if you had a vacuum leak, all by itself, there is no fuel in that air, so it's dry air. When dry air is introduced,
Two or three things are gonna happen;
1) If the idling engine is already rich, yes the rpm will pick up. Then you will go and back off the speed screw, closing the transfers. The engine will go lean, and so you will increase the fuel from the mixture screws. So now it is idling half decent. But as soon as you put it in gear, she will want to stall. You didn't mention any of this.
2) If in the idling engine, the AF ratio is already normal, then extra dry-air will slow it down and the exhaust will get stinky. You didn't mention this either.
3) If the engine is idling lean, and you introduce dry air, it will slow down and run rough.
In ALL cases, the idle speed can be changed with idle timing. But, in the narrow range you are likely to be working in, it will affect the idle quality very little.
But if the flyweights inside the D are stuck in full advance, or the Vacuum advance is engaged, then you will also have a very fast idling engine., with usually no adverse idling characteristics. And the telltale of that is the increased torque, which chirps the tires when going into gear.
Some of these maladies have already been mentioned; but now you know the why of it.
Happy hunting
