Duane S. Crowther discusses late reminiscences of Luman Shurtliff, Orson Pratt, and Lyman Wight that Joseph believed the Saints would move to the Rocky Mountains.
Duane S. Crowther, "A Study of Eschatological Prophecies Found in the Scriptures and in the Works of General Authorities of The Church of Jesus Christ of Latter-day Saints," (M.A. Thesis, Brigham Young University, 1960), 18-19
Luman Shurtliff, upon his arrival in the Great Basin in 1851, also made it clear that the Prophet had definitely contemplated a western exodus:
We got into the Salt Lake Valley, September 23, 1851, thankful to the God of Heaven that I and my family were in the valley of the Rocky Mountains—here where the Prophet Joseph Smith had said thirteen years before [in 1838] that the Saints would go if the government did not put a stop to the mobbing and the persecution of them.
Orson Pratt also testified that the Prophet had anticipated the exodus long before it transpired. In a public meeting held on April 26, 1846, in the early days of the exodus, he stated:
It is eight years today since we all came out of Missouri. Before that time Joseph the Prophet had this move in contemplation and always said that we would send a company of young men to explore the country and return before the families can go over the mountains; and it is decidedly my mind to do so.
His testimony was corroborated by Lyman Wight, who stated in a letter to Wilford woodruff that “such a mission was even talked of while in [Liberty] Jail.”