We just bought a 2001 Durango 4.7L with 160000 miles. The idle will go up to about 1500 to 2000 after the engine was warm and the transmission would start to down shift like it was under a heavy load.
I replaced the Idle Control Valve and cleaned the throttle body. I also found that the vac line from the driver side of the throttle body had a split in it so I replaced it, but I still have the problem.
I live in Indiana and the temperature only reached a high of 20 recently and on those days we did not have any problems.

 

Clarify please, your use of the term 'idle' in the second sentence. You say the idle goes up to 1500 to 2000 but then mention the transmission shifting down, which only happens if you are driving. Do you maybe mean 'engine RPM' instead of idle? Idle happens when you are not pressing on the gas and usually when you are stopped. Idle means, not in motion. LOL. I think you mean engine RPM, as in you're driving when it happens and it sounds like your overdrive is switching off and on. This will typically jump the RPMs up and the Durango will feel sluggish like it has a load.

 

I am sorry if I was not clear what I meant was that it will idle between 1500 and 2000 after the engine is warm and when you are driving it will down shift like the engine is under a heavy load. When the outside temperature was in the teen's we did not have the problem but know that it is in the 30"s the problem has gotten worse. I hope I have made my self clear.

 

I think the issues might be separate. Have you checked the transmission fluid when it is warmed up, like after 15 - 20 minutes of driving? Make sure it is full. When the transmission fluid level is low the low fluid pressure greatly affects the timing and strength of the shifts. Also, can you recall the last time the filter and fluid was changed? A dirty and clogged filter will also greatly affect the above processes.

As for engine vacuum, I think you were on the right track with finding and solving vacuum leaks. You said you found one cracked line and replaced it. It stands to reason that there could be more cracked rubber connections...and possibly bad check valve. Might not be a bad idea to check all visible connections of the vacuum system. Sometimes spraying brake cleaner on and around the connections and intake will reveal a leak as the RPMs spike from the fluid being sucked in through said vacuum leak.

These things are easy and very cheap and it might help you locate the issue before throwing money at sensors and parts that 'might' be an issue. I'm sure a few people will suggest the plenum gasket is leaking, and if you haven't I think it needs to be checked.

To check, disconnect and unbolt the throttle body, and using a flashlight, peer down into the intake and see if you see oil residue or pools of oil on the steel pan which is the bottom of the intake keg itself. If you see oil in there, specifically in the corners by the intake ports...then the gasket is bad and needs to be replaced or more problems will follow.

If the pan inside is clean and dry then the gasket is fine and probably not related to your current issues.