This is definitely a fuel sending unit issue. Available at the dealer still for between $50 and $100.

Not rebuildable? I don't know. <img src="/forums/images/graemlins/wink.gif" alt="" />

At mitsubishiparts.com:

Senders, Fuel gauge sending unit
Montero
List Price : $43.00
Your Price : $32.77

<img src="/forums/images/graemlins/smile.gif" alt="" />

1987 Raider - Roxy
1988 Mighty Max 2.6L Turbo - Pearl
1997 Mountaineer V8 - Freddy
2000 Excursion V10 - Freya