The original Book of Mormon text uses exceeding as the adverb modifier for adjectives and other adverbs. Beginning with the 1920 edition, LDS editors started to add the suffix -ly to these adverb forms (thus exceedingly). The process was systematically completed in the 1981 edition. See the discussion under exceeding in volume 3.
This use of exceeding in front of adjectives and adverbs is characteristic of Early Modern English and is found throughout the King James Bible. For instance, in Matthew 5:12 Jesus declares: “rejoice and be exceeding glad”. The corresponding Book of Mormon passage also originally had “be exceeding glad”, but was subsequently edited for the 1981 LDS edition:
Summary: Wherever supported by the earliest textual sources, restore exceeding as the adverb modi- fier of adjectives and adverbs.