I have some bad news for you - working on the 'saddle' tank on the AWDs is a real PITA:
http://www.brickboard.com/AWD/index.htm?id=1362154&show_all=1

I think I read somewhere - maybe MVS - that there is sometimes a problem with the hose clamps on the fuel filler hose... you'll never get to it without either dropping the tank ($$$) or cutting holes (PITA again).

I'd probably cut some well thought out access holes. By "well thought out" I mean, do your best to figure out the parts you're trying to access and where they are, and also keep 're-assembly' and sealing things back up in mind - any brackets or holders or anything like that will have to go back in it's exact same location when you're done, and any 'tubular' or 'structural' shapes might be dangerous to cut out. Also there will likely be either wiring or plumbing of some kind lurking directly behind the sheet metal so try to use the 'slow' cutting methods like chisels and snips as opposed to grinders, etc.

I would recommend some serious digging before you start cutting holes or hand your car over to the $tealer.