Need major help!

The 300f4 AFS + tc14e is as good as it gets at any price. I agree with Greg...if you're gonna be on a tripod, the replacement foot is a very wise investment.
The Sigma 100-300HSM f4 + tc, 50-500HSM, 80-400OS, and Nikkor 80-400VR are all within your range.
Another possibility if you are open to MF lenses is the 400f3.5 AIS + tc-301. Outstanding combo but your support rig and technique are key. I reluctantly sold mine, but it was great (BTW I never saw the CA that others have mentioned ). Of course handholding is not going to be something you'll want to do with this one...at least not with any regularity.

 

Jen:
You may want to consider renting a 200-400mm VR or a 5/600mm AF-s II, depending on how close you can get, the amount of light available, and the quality of image you're seeking. Yes, shorter tele's will work, but your odds of getting the 8x10 of a well-exposed, crisp shot increase substantially with one of the super tele's. Given that this may be a 'once in a lifetime' opportunity, renting the best equipment seeems justifiable.
Another thought is to purchase the 500mm P4 (manual focus), which is just above your budget.
My experience has been that compromising focal length leads to dissapointing shots of small wildlife, but my expectations of shot quality may differ from yours.
Best of luck
Dave

 

Though I havent shot with it myself I have seen many pictures with the sigma 50-500mm ex lens that I have to say I was very impressed with. just a thought.

 

For hand held the Nikon 80-400 is great and since they are a "nesting pair" the slow AF should not be a problem. Otherwise if you have the tc14e (type I or 2 i think) the 300 f4 is a great lens from all accounts.

If your TC is the Kenko then there are a few here that have actually used it on the Nikon 80-400 with acceptable results given enough light. Also the Sigma 50-500 is made with a limiter that stops you from going down to 50mm so you can put the Sigma TC on it, for Sigma lenses it is best to used Sigma TC's. It turns it into a 700mm, but you need a lot of light.

The only one of these that I have owned is the 80-400, and I did try the TC on it a few times.

 

I had the 80-400VR when it first came out and I was less than impressed with the image quality, especially at 400mm. I recently bought a new 300 f4 and so far I am quite pleased with the image quality, even when using the TC1.4eII (420mm). While I advocate buying only new when it comes to lenses with AFS and/or VR, you might consider a used one if you can find a good deal (There was just a good one here not long ago...). Rental, as mentioned, is also a viable option, unless you just want to use this as an excuse to satisfy your lens lust  

 

300f4 + converter for your budget, period!

 

I agree with the 300mm AFS f/4 + teleconverter. I have owned the Bigma i.e. Sigma 50-500 and it's very good, but must be stopped down to f/8 for sharpness (at least mine required this). I currently use the 300mm and also use a 500mm f/4 AFS, which is a good all around big lens. I also like the idea someone posted about renting a big lens. Not a bad idea, because you may only need a 500mm for a short period, and you get to try it out.

 

Before I would rent I would think about buying used and reselling if you can. Might be cheaper. I have a trip to Alaska planned for this summer and I'm not sure if I'm going to buy, rent, borrow, or buy for resale myself.