COUNTRY Willie Nelson & Waylon Jennings: ''WWII.'' (RCA 1-4455.) No two people today can give that laid-back country feeling as completely as this pair. Their contrasting vocal styles, with Waylon's deep voice and Willie's tenor, enhance their work together. Mr. Jennings dominates the record as far as solo efforts go , but the duets are Willie & Waylon at their best. The remake of Otis Reddings's ''Sittin' on the Dock of the Bay'' embodies its own special richness and reflectiveness. Bouncier tunes such as ''Mr. Shuck and Jive'' and ''Clayton Delaney'' give one the urge to throw on some cowboy boots and head for the range. Two of the album's most beautiful songs are ''Last Cowboy Song,'' written about the lost art of being a cowboy, and ''Heroes,'' written through the eyes of a young man who loses his heroes just when they are needed most. The album is great for country fans, but should also attract rock-pop fans who enjoy Willie Nelson.