THINGS ARE SO MELLOW on Jim Guercio's Caribou Ranch, those 3,200 acres of secluded headland lying right outside Denver, that even the birds seem to be humming 'Rocky Mountain High'. Lakes splashed about the vast acreage, trees dotting the rolling hills, all relax in a brief but sensational summer storm which sends rain drops hurling in the 80-degree weather. Don Felder, the newest member of the Eagles, looks out at the downfall from under the lodge overhang, his reflective sunglasses no longer necessary and his cup of hot coffee now a welcome tonic. Felder seems comfortable here among the log cabins, the horses and the studios.