The LDS text has consistently eliminated the archaic word burthen in favor of the modern burden. The Oxford English Dictionary gives many examples of burthen being used in the 19th century (especially in the first third of that century):
1803 Arthur Wellesley a beast of burthen
1812 John Wilson the burthen of despair
1825 John Nicholson to carry a greater or less burthen
1827 John Keble thine own sad burthen
1832 Harriet Martineau without burthening the parish
1833 Isaac Taylor the burthen of the dull echoes
1834 Charles Hoffman the burthensome steamboats from New Orleans
1876 Edward Freeman certain feudal burthens
The King James Bible, as currently printed, consistently has burden (101 times), but in the original 1611 printing, seven of these were burthen (from 2 Corinthians 8:13 through Galatians 6:5). In Shakespeare’s works, burthen is favored, although burden also occurs, with 63 occurrences of burthen versus 10 of burden (according to a standard electronic database). Nearly all the occurrences of burden in the original Book of Mormon text were burden (18 examples, including five that are found in quotes from Isaiah), but there were originally three occurrences of burthen:
All three of these occurrences of burthen have been replaced by the corresponding form of burden in 20th-century editions of the LDS text, beginning (in two of the three cases) with the 1906 largeprint edition. The critical text will restore the three original occurrences of burthen but leave all the other cases of original burden. In each case we let the earliest textual sources determine the original reading.
Summary: Restore the three original occurrences of burthen in the Book of Mormon text: Jacob 2:23, Mosiah 6:7, and Alma 62:29.