Idling too fast

Engine Performance problem
1996 Ford Ranger 6 cyl Four Wheel Drive Manual

I have a 1996 ford ranger xlt, 3.0 v-6, 4x4 manual trans. Replaced motor, 129,000 miles on truck. It has high idle while driving, between gears. Any suggestions?

My ranger has been idling high for about two months now. I took it to a mechanic who said I had two bad fuel injectors. I bought new ones and replaced them, but my idle was still at about 2500 RPM wheras normally it would be about 1300. Next I was told that it could possibly be the IAC valve and that it could be bad. So I ordered a new one and replaced it. So now I have 2 new injectors and a new IAC valve and it still idles high. Also its running alot louder, and im not sure where this comes into play but its definately noticable.
I took it to yet another mechanic, and he says its an intake manifold leak and that I should replace the gaskets. However, its only idling high when its in park and neutral, when its put into drive or reverse it drops back down to normal.
Ive also checked all the vacuum hoses for a leak and made sure there wasnt a snag on the throttle cable.
Any Suggestions? Thanks.

Will do, thanks. I`ll let you know.
Also. Could it be the Intake manifold gasket? Also I have the same kind of oil leak. So could a bad intake gasket cause a leak and also make my idle high?

I have a 96 Ranger with a 3.0 V-6.
When you crank engine initially it has high idle, no tach to see rpm but it does slow down after a minute. Just seems abnormally high. Not used to small truck engine.
Also when drive or reverse is engaged, it is a firm jar.