'97f150RKB

Hey guys, Im having a slight issue with my truck. When its in park or neutral it idles just fine, however, when I put the truck into any gear it bogs down majorly. The truck is an XL model and does not have a tach. I recently removed the intake silencer and did the "gotts" mod to my intake and that is when the problem began. When I take it back out of gear it idles up a good bit and then drops back down to normal idle. I am wondering if it is maybe the idle air sensor or the throttle position sensor. I already reset the pcm and saw no change. I checked for new codes as I have a CEL and found nothing that wasnt already there. The truck has around 250,000 miles on it and still runs very strong until this problem arose. Any help will be greatly appreciated! Thanks!