Before I start digging what vac lines are behind the dash. I hear a leak at idle,during acceleration it cuts down but at cruising speed it comes back. I have cruise control but haven't tried yet. Just got her yesterday.. Am not sure if 4wd is engaging either.
There are vacuum lines going to and from the 4wd switch on the dash, and some of the heater/defroster controls use vacuum. Those are places to start looking for disconnected or rotten lines for a vacuum leak inside the car.
Anything for cruise control
The only vac line for the cruise stays in the engine compartment. Cruise control, whether factory or added on, only has wiring inside the cabin.
Well, except for that silly cruise vacuum bleed-off switch on the brake pedal. But it only has vac to it while the cruise is engaged.
Unless it's hooked up wrong. Bad brake booster will do that too.
Once I noticed when she was parked for 4hours the brake pedal was solid the first time I stepped on it and I haven't stepped on it at all when it was parked. She is running great since I found and pluged the leaks I could find. A manifold vac line was off. I could hear the leak inside the car from under the hood. That was the reason for the post,it sounded like it was behind the dash.