west coast route

unless you're driving out of mexico,you don't need to renew your us plates for the 4 years of your temporal

Thanx Pedro.

I'm trying to figure out how to sell my car here at home before I leave and still get me, my furry friends and some possessions down there. I'm considering renting a car in Portland, driving down and maybe exchanging for different rental at the border..then buy a car there..any thoughts?

That's probably a good idea OR you and your furry friends fly down and you ship the possession's you need to bring. I would cost out both ways and see.

Much depends upon your INM status. If you have already been living in Mexico and have a visa, you could drive your US car to the city nearest the border with a CarMax and an airport serving your Mexico destination. Drive down, board the dogs, fly to your Mexico destination, buy a car, drive back to the US, pick up the dogs, rest a bit......etc.
Otherwise, minimize the belongings, store them, fly to Mexico with the dogs and worry about the belongings on a later visit north. Simpler? Do not forget to get visa approval at the nearest Mexican consulate before you leave.