Mande languages are well-known for their rigid SOVX word order: verb phrases cannot accommodate postpositional phrases, and all oblique arguments must appear after the main verb. This study explores, based on data from Wan (Southeastern Mande), new evidence for syntactic constituency that is essential for developing a formal account of this typologically unusual pattern. First, I show that previously unexplored tonal evidence rules out argument raising accounts. Tone is sensitive in Wan to prosodic phrasing, which is in turn closely related to syntactic constituency; the way postpositional arguments are prosodically integrated into the clause points to their unusually high, clause-level attachment. Second, I argue against a base-generation analysis, which would require a serious modification of the Projection Principle and locality of selection. Third, an analysis based on obligatory extraposition is discussed as the remaining option in transformational frameworks. While accounting for both semantic and tonal evidence, the extraposition account has to rely on a highly unusual kind of filter to rule out all structures where a PP argument appears clause-internally. Accounts postulating such idiosyncratic filters can hardly be considered satisfying, as they merely model constraints on surface structure, without deriving them from underlying structural properties. The obligatory argument extraposition of Mande languages receives a more elegant explanation in constraint-based, surface-oriented theories, which do not need to introduce special devices to handle the basic word order of Mande languages. I illustrate this with a sketch of an account coached in the framework of Lexical-Functional Grammar.