I have idle probs with my 87 jeep cherokee i dont know what it could be?

i think it may be my iac valve or my tps but idk how to tell if those are bad and any ideas what could be wrong it idles fine in park an neutral but in overdrive it has problems and i had oil back up trough my intake any ideas why that happened. and by the way my jeep is a 87 cherokee laredo inline six 4 litre. aslo if you have any ideas bout any cool mods lemme know im fixing it up this vehicle was just givin to me