TREATED as a single unit is used as singular.Example: The committee has agreed to the PROPOSAL. Here, the committee is one single unit.A collective noun treated as separate individuals is used as plural.Example: The army have continued fighting bravely. Here, each individual of the army is considered separately. Although, one can alter their perception of the group as a single unit or of separate individuals in a group, in order to use the verb.However, in this sentence, the jury has been described as being 'divided' in their opinions. This means not all 'individual' members are in agreement, THUS making it important to consider individual opinions in the jury. THEREFORE, the sentence is: The jury were divided in their OPINION.