The NASB, KJV and RSV translate the word רבה as "greatly", which is close to a linear (i.e, "literal") translation. The disadvantage is that "I will not be greatly shaken" could be misunderstood as "I will not be greatly shaken, just a little stirred".
OTOH, the NIV and NRSV avoid the possible "stirred, not shaken" misunderstanding and emphasize the use of רבה as an emphatic or superlative, by translating רבה as "never". This results in a translation that is closer to the meaning of the MT in current US English.
Both translation approaches translate רבה as an adjective modifying a verb, shaken, which is unique in the MT. The only other similar verse is Psalms 110:6, where the common translations see רבה as modifying ארץ, a noun, although when reading the MT it appears that רבה modifies מחץ, which is apparently a verb although possibly part of a noun phrase, מחץ ראש.
There is no similar usage of רבה in later historical layers of Hebrew. The usage in this verse therefore sounds quite awkward and out of place and is therefore worthy of commentary. In fact, the classical commentators, RASHI, Ibn Ezra, MALBIM and others make various attempts to explain this usage.
The intent might be:
אַךְ-הוּא צוּרִי וִישׁוּעָתִי רַבָּה מִשְׂגַּבִּי לֹא-אֶמּוֹט
which in English would read:
But He is my great salvation, my rock, I will not tremble
but the word is instead placed at the end of the verse for several likely reasons:
- in order to rhyme with הַדְּחוּיָה at the end of the following verse
- as a lead-in to the following verse
- not placing it at the end would break the chorus line repeated in verse 6 (MT 7)
As a lead-in to the following verse, רבה modifies the following verse to give the meaning:
How exceedingly long will you attack a man?
instead of (NKJV):
How long will you attack a man?
In any event, this is clearly word-play, seen commonly in Psalms.