IIRC, the 1999 Highlander models have the multi-mode 4wd, meaning you have a 2H; 4H (center diff unlocked, aka AWD); 4Hl (center diff locked, aka 4WD); and a 4L. I am sorry if my memory is serving me wrong, but this should get you going.
2hi to 4hi:
while traveling in a straight line going no...