NADA trade-in is "supposed" to fall right in line with wholesale. Classified person-to-person pricing is usually a bit higher and dealer used car pricing is higher again (but you get 3 month warranty).
In these odd times, the only clear usable indicator of true wholesale value is the Manheim auto auction database. It lists the sell price of stuff nationwide weekly.
Right now, pre-00 Yukon/Tahoes are running about a grand UNDER nada trade-in for wholesale numbers (at least in the southeast).
You MIGHT be looking at a truck worth 14k, 15k if it is the cleanest unit known to mankind.
Is this currently your car? Do you already have a loan on it? I am confused? Is this a lease buyout you are talking about (which would make the most sense)?