Montero 03 M-astc Question, M-ASTC Options

[Data08]

Hello,

Im new to the montero world, well my dad had a 93, but Im new. I have an used 03 Montero. The problem im facing is that, as I understand it, the M-ASTC light should be on always as it says in the manual, but mine just never comes on, exept when starting the engine. Is this normal? Also when Im about to hit 60 MPH I feel as if the engine vibrates, as if it was strugling to stay on 60mph. I've noticed that everytime the tachometer hits around 2,100 rpms the engine or trany vibrates and makes a vibrating noise. It does not happen when im in the city driving at around 35mph... but it still always vibrate at 2,000rpms...

Please help. I know this is probably a 4WD newbie question.

Thanks.