Vehicle idles up an down

How long since a tuneup? Any applicable trouble codes?
The generic info for rough idle on any vehicle is to check for a leak of some kind, fuel--egr--vacuum, something of that nature.

Valve stem seals. Oil is thicker when cold (hence no smoke when cold) Vlave stem symptoms are when engine at idle (high vacume) oil is pulled through the guide seals, when you take off from lights get bit of smoke until oil is burnt. Ring symtoms is it would be blowing smoke all the time especially at higher rpm for extended time ie: driveing. Have you thought of trying a thicker oil? what oil grade do you currently run?

distributor got a problem

you didnt say if it was happening only during the initial warm up of the vehicle or all the time. if it is just during warm up this is not too unusual as long as the engine isnt racing. if it is just a "high idle", say arround 2,000 rpm's its normal. the idle would usually go down as the vehicle reaches operating temperature. when you put the vehicle in gear, you are actually loading the engine slightly, thats why the idle comes down then. when in park the engine has no load on it so it is free to rev a little higher at these times. if it a at a high idle constantly, then i would start looking for sticking throttle cable, binding throttle plates, ect and remedy the problem. this should not have anything to do with the dual exhaust.

Possible Idle air control, simple to change on the throttle body

check ignition timing

Check your oil stick and make sure the oil is at the right level.

You have one of two very common problems.
Number one may be a stuck open EGR valve
Number two would be a failing Idle Air Control valve (IAC).
Try this: Open the hood & locate the EGR valve, close to the firewall, behind the intake. Find & take with you a small hammer. The next time that it happens, open the hood & tap on the EGR valve. If it is sticking "open" the tapping may dislodge the carbon particles that are keeping it open.When the EGR valve is open, exhaust is getting into the intake at idle, and this will def stall out the car.
There is no repair to the IAC valve, it would need to be replaced if it is bad.
One of these two scenarios will cure your problem, I see it every day.

Interesting problem... I myself own an 88 Ford Bronco II 4wd 2.9L V6. I have issues with my idle reving up to 1500 to 2000 rpms at stop. I have not yet resolved the issue, however, I do believe that sometimes electronic components are to blame...such as the brain or "Mother Board" of the vehicle. I replaced the "Brain" on two previously owned vehicles with instant results. I would suggest the local junkyard for a try...only as a last result. The "Brain" is located on the passenger side along the Interior wheel well. If you look along the front passenger side wheel well inside, beneath the plastic, vinyl, etc....you will find a metal enclosure that is comparable to a computers cd drive or hard drive........Approx. 4" x 6" give or take. The Board will have a wide slot plastic attachment plugged into it...very similar to an IDE slot on a computer.
This is the Brain......for your Bronco II.
Buying a used Brrain is a Gamble ...
You could get a bad brain!!!

BUT...You could get an instant repair!!
Who's to decide?
I had an 1986 BMW 325I
Suddenly One Day....
It started Reving up and down erratically....I was at a loss.

Luckily, I had a friend who was a BMW mechanic, and informed me that there was a relay in my glovebox that controlled my idle. I took a chance and tried replacing the relay with one I purchased at the junkyard...although they wanted $50 for it...I was willing.....and the new /USED idle conrol relay turned out to be an affordable fix. Thankfully

sure, that could be it. I would also suspect the fuel filter as well.