racing bronco

first off id like to say hi to all you guys. youve got another newb in your midst.

my 1990 bronco xlt with a 5.0 and e4od will not idle down. intake manifold vacuum test did not indicate a leak. ive vacuum tested every vacuum line and found nothing leaking. codes pointed to tps and iac both of which i replaced. i also replaced the throttle body to intake manifold gasket. plugs wires cap and rotor. nothing has helped.
in park it idles at either 1500 or 2100 rpm. put it in drive it will idle 900-1000 rpm. in reverse it idles around 850-900 rpm. what in the world is going on ? should i be looking at my trans instead of my motor ?