Wow, going from a D70 to a D750/D800 is going to be quite a huge step! I’m just curious as to the reason that you want a full-frame sensor. Is it due to the extra megapixels/resolution, shallower DOF, or just because it’s a better camera?
Depending on your answer, there are definitely other options available in the high-end APS-C sector of the market. The Canon 7D Mk II, Canon 80D, or the Nikon D7200, or even the Nikon D500. The Nikon D610 is another full-frame option, as well as the Canon 5D Mk III and 6D. Then there’s the lesser known market of Pentax. The Pentax K-3/K-3 II (APS-C sensor) are highly regarded for their image quality and sturdy build and weather resistance. And if you’re sure that you want a full frame, Pentax just released their full frame camera this year, the Pentax K-1! From the few people who have received their pre-ordered ones, it looks like it is as good as or better than the Nikon D810 (which is an upgrade from the D800.) The best part is, it fits in your budget brand-new at only $1769!
One thing to consider, is what lenses you may already have for your current Nikon camera. Do you have a lot of money invested in good glass for Nikon already, or are you just shooting with kit lenses? Almost any good photographer will tell you to invest more in lenses than in the camera. You can’t get a bad camera these days, it’s the lens that will have a big factor in how “professional” your pictures are. Camera bodies wear out in 10 years or less, (as you found out), but a good lens can last you a lifetime.