The group in the campaign I DM has a similar problem. While they have an arcane caster, he is a multi-classed sorcerer, and his sorcerer levels have always been about 1/2 of the average party level. Currently, the party has an average level of roughly 12, and he has 7 sorcerer levels. That means that, while one could expect the arcane caster to be slinging around Disintegrates and Acid Fogs, he is instead still slinging low-powered Fireballs and Magic Missiles.
That being said, I really don't feel that the party misses a strong arcane caster much. They have a lot of muscle-power, and they can collectively take a beating and dish one out. They have redundancy in divine magic (high-level druid, high-level cleric, paladin, multi-class cleric), which allows them to continue pressing on where others might have to retreat to heal, and it also grants a surprising amount of offensive firepower. When you have two party members casting Flamestrikes, you don't really miss Chain Lightning.
I think the makeup of your party is going to be a significant factor as to whether or not you will miss arcane magic. Noting that your party only has one primary caster, period, I think that the group will find certain classes of challenges difficult, especially those involving non-standard movement (underwater, flight, through a wall, etc), and those involving large numbers of enemies.
Looking at your party, I would make the following suggestions. First, make sure that the cleric always has a Dispel Magic handy. Second, make sure that the ranger has Entangle memorized, have the Drow use his Darkness ability liberally, and have the cleric prepare spells that divide or neutralize areas of the battlefield. By doing so, they can help even the playing field when they are badly outnumbered.
For mobility, buy some potions of flight and waterbreathing, and hope you don't run into too many encounters where that is an issue.