SHIFT is a novel framework combining uncertainty modeling with informative priors derived from automated rule extraction. SHIFT reformulates trajectory prediction as a classification task and employs heteroscedastic spectral‑normalized Gaussian processes to disentangle epistemic and aleatoric uncertainties. Informative priors are learned from natural language driving rules using a retrieval‑augmented generation framework powered by a large language model. Extensive evaluations on the nuScenes dataset demonstrate that SHIFT outperforms state‑of‑the‑art methods, particularly in complex scenarios such as intersections.