i had one of these do the same thing. jack passenger front wheel off ground leave the others on ground. try to turn the pass wheel back and forth by hand while watching your steering arms. especially your pitman and idler arms. my idler was worn and letting the steering arms move up and down. this is very common with the ifs front. it would pull one wheel in, then the other when its locked in 4x4. that is the open diff switching back and forth. when its in 2wd there is nothing pulling on the front wheels to make them go L to R. have someone stand in front while you drive toward them to see what its doing.
if it is different gears front and rear. you should hear and see some serious tire slipping, just when idling around. for your sake i hope its just the idler arm. its much easier to fix.