I agree with saving your Excursion if possible since they don't make them anymore. But, only you know what condition the rest of the truck is in - if the body is rotting out forget it. We live in the snowbelt of Lake Erie and salt is used here extensively with serious consequences to our vehicles.
Oh - and I would advise against buying a junkyard engine. We did it once and after all that work to remove and install - the main bearings were shot - had great oil pressure and sounded smooth till it warmed up.
We have taken to buying new (used) vehicles down south and if needed replace the engine with a Jasper. We have 2 daily drivers that have Jasper engines in them - one from 2001, and one from 2008. We've had good luck with them and would do it again.
I'm not talking about our tow vehicle, but if we do have engine trouble with it we might just get another Jasper... Unless we decide to spring for an F150 Ecoboost, that is. ;)