When you say a $20K Xterra, I assume that you are talking about a used Xterra because a 4x4 Xterra msrp is in the upper 20's. If you are buying used, especially a 4x4,there is no guarantee that the tranny will not drop out of what you buy tomorrow. And on top of it all, you will be driving a rice burner.
My opinion is to fix your tahoe because you are going to fix it now or later and depending on how much later, it is only going to get more expensive. Take it to the dealership for the taillight issue and have them troubleshoot the AC. The AC may be an issue that is solved with new parts like compressor, condenser, etc. It may even be a burned out diode in the controls. If that is the case, buy the parts and replace them yourself. People are so intimidated by AC's and it is a really simple system with few mechanical parts. I do admit that there is little that a driveway mechanic can do when something detrimental happens to an AC, but more often than not it is a problem that is easily remedied by a component replacement.
As for the other issues, roll on over to autozone, buy the parts, spend half a day and replace them.
If you are going to keep it, you have to fix it; if you are going to sell it, you have to fix it unless you are willing to make a significant price cut.