Ibn Ezra

Serve (עִבְדוּ - ivdu): [This command is] the opposite of their words [in verse 3], "Let us break (nenattekah) [their cords]." And the service of HaShem [should be] out of fear (le-yir'ah) of Him, and [so as] not to transgress His command (lit. His mouth). And rejoice (וְגִילוּ - ve-gilu): If joy (gil) comes, do not forget that you should tremble (tir'adu) out of fear of HaShem. [Explaining the combination "rejoice with trembling"]. And some say (v'yesh omrim): [That the imperative form] rejoice (gilu) is like [the future/imperfect form] "you will rejoice" (tagilu). [This follows] the pattern of [sequences where imperatives imply a consequence or condition, like (Deuteronomy 32:50)] "Ascend (aleh) ... and die (u-mut) on the mountain," or [like the blessing in (Genesis 1:22)] "Be fruitful (p'ru) and multiply (u-rvu)," which is a blessing upon the fish of the sea. And the meaning [according to this second view] is: That you will rejoice (tagilu) if you have [first] trembled (ra'adtem) out of [fear of] HaShem.