If you don't want opposite rolls, a direct fix for the shield: It gives a flat deflection bonus to your AC (as the Shield Ward feat from PHB2). Maybe add a reflex bonus in some situations or something.

As for using BAB as defense, I'll assume you checked the Defense and Armor as DR rules from Unearthed Arcana (available in the hyper text site here http://www.d20srd.org/srd/variant/ad...fenseBonus.htm and http://www.d20srd.org/srd/variant/ad...eReduction.htm).