I replaced the sending unit on my '92 for much less than the figure quoted to you. Are you sure you're getting a sending unit quote, and not a fuel pump assembly price?
I want to say I spent under $100 for the sending unit from my local dealer, but I'd have to drag out the '04 receipts to be sure. Just talked to my buddy at the parts dept, he states the sending unit is $62.50 and the pump is $491
Part numbers-sending unit MB571603- Pump is MR208665
This was looked up for a 3.5 '95 Montero, I don't know if the LS with the 3.0 liter is different. Hope this helps you out-- PS, have you removed the access cover on the floor in the cargo area for the pump/sending unit to get a better look? It's under the carpet/floor material