JOEL - There is no evidence of God-sanctioned plural marriage recorded in the book of Mormon. There are plenty of instances however, where lustful men took it upon themselves to practice a form of polygamy as seen in the following scriptures:

Jacob 1:15
And now it came to pass that the people of Nephi, under the reign of the second king, began to grow hard in their hearts, and indulge themselves somewhat in wicked practices, such as like unto David of old desiring many wives and concubines, and also Solomon, his son.

Mosiah 11:2
For behold, he did not keep the commandments of God, but he did walk after the desires of his own heart. And he had many wives and concubines. And he did cause his people to commit sin, and do that which was abominable in the sight of the Lord. Yea, and they did commit whoredoms and all manner of wickedness.

Ether 10:5
And it came to pass that Riplakish did not do that which was right in the sight of the Lord, for he did have many wives and concubines, and did lay that upon men's shoulders which was grievous to be borne; yea, he did tax them with heavy taxes; and with the taxes he did build many spacious buildings.

There is evidence in the Book of Mormon that God would allow the practice under certain circumstances as shown in the following verses:

Jacob 2:24
Behold, David and Solomon truly had many wives and concubines, which thing was abominable before me, saith the Lord.
Jacob 2:27
Wherefore, my brethren, hear me, and hearken to the word of the Lord: For there shall not any man among you have save it be one wife; and concubines he shall have none;
Jacob 2:30
For if I will, saith the Lord of Hosts, raise up seed unto me, I will command my people; otherwise they shall hearken unto these things.

In verse 24 and 27 God condemns the actions of David and Solomon who took wives for the wrong reason and without God's permission. But later in verse 30 God explains that it would be allowed if he wanted to raise up a righteous seed, such as was the case in the early days of the restored church.