The main downside of lower gears is higher rpm on the hwy, which means more noise, slightly more engine wear, and the possibility of lower mileage (or higher if the gearing isn't right).

I think the key here is to build up your Jeep and particularly your drivetrain as an entire system. In other words, don't just guess at the gearing. Match the gearing to the existing trans, tires, and engine mods. For example, most engine mods move the power band up in the rpm range at the cost of lower rpm torque and so lower gearing is called for. Auto trans requires different engine build than manual and each require different diff gearing

Anyway that's the theory. Pretty sure my solution isn't optimal yet.

I have a 3spd trans, so lower gearing means screaming down the hwy... I opted to restore the original effective gearing for th stock tires by going from 29" tires to 33" tires and 3.31:1 gears to 3.73:1 gears. Kind of high, yes, and hurts power a bit.

So if you have OD it'd be no problem to find lower effective gearing than this without screaming at 75.

For what it's worth, check the gear calculator at http://www.4lo.com/

Also, check out this article on gearing:
http://www.high-impact.net/transmission_and_gear/Systems_Approach.htm

Michael