There's some overlap. There's a ton of info on this thread:
4WD won't disengage on 99 4Runner LTD - Page 28 - YotaTech Forums
There's a few differences between 99-00 Multimode and 01-02: Yours is vacuum activated, mine is electronic. Yours is 4 pin, mine is 6. Your front diff actuator has a removable sensor, mine is integrated into the actuator. You can easily pull your TC actuator, and based on the lack of responses to this thread, I don't think I can pull mine. You have an ADD relay, mine doesn't.
Here's what I would suggest:
- Check your ADD relay.
- Pull the 4WD sensors on the back of the T-case and check for continuity, there's a good
video here.
- Also pull the sensor on the front diff and check that.
- Try and activate both actuators via the vacuum tubes. If they work fine, your problem is likely upstream (in the VSV or somewhere in your electronics).
- check to make sure the 4WD switch on your shifter is working correctly (via continuity test.)
Other possible problems:
There are two sensors on the top of the TC, one for 4L and one for neutral. I'm not sure what sort of issues they cause, but you need to drop the TC to access them, so I haven't looked into that yet.
A lot of people have issues with the TC actuator - might be worth it to pull it and check for corrosion and water damage. There's some debate about "timing" when reassembling, I'd do a lot of research before taking that apart.
A fair number of people have said the ADD actuator went bad. That part is $300 new, but I got one for $20 last week at the junk yard.