Upload & Sell: Off
A D700 and 28-300 will still take excellent pictures... if size/weight/money tied up in your glass has you itching for something lighter, and you don't need the super-shallow depth of focus your 70-200/2.8 affords you, and you don't use the macro capabilities of your 105/2.8, and you have no need for ultra-wide angle shots from your 16-35, go for it.
If on the other hand, you need the capabilities that the pro lenses in you bag afford you over the 28-300, then a kit sell-off would be a mistake. It really depends on what you shoot, under what conditions, and if you could see yourself being satisfied with a jack-of-all-trades, master-of-none 28-300.
For me, if I were looking to save on size/weight and go with a do-it-all zoom lens, I'd go back to Micro 4/3, get a Panasonic 14-140mm and a decent Micro 4/3 body like a GH2 or OM-D and call it a day. I just sold that kit and re-bought a D700 to do pro-level work again though... so would I go for a 28-300? Nope. I'd shoot with the pro kit, and be selective of the lenses I bring with me on a trip. Good luck in your decision.