2003 Montero is wandering abruptly, front end is good and tight .Dashboard warning light of tilted vehichle (2 of them) is on.Could a malfunctioning speed sensor on rear left wheel cause this

 

I would check your ball joints. Mine started going out and did the same thing, but when I did the ball joint test by lifting the front end and grabbing the wheel to shake to check for play, there was no movement. Pulled the knuckle and those suckers were about as loose as a dead goose.