[comptiger5000]

^ BAD idea. With the exception of the Rubicon, the front diff and axle shafts are noticeably weaker than the rear. If you want the same benefit, pull the front shaft and leave it in 2wd. It'll still drop the rotating weight a bit. It really can't do much, as both diffs and such are still spinning. Lockout hubs for the front would help a bit more.

As far as weight, you're right that an early base model TJ with a soft top, 4 cylinder and a 5 speed is just under 3100 pounds. However, that's with the smallest stock tires available, and no options. Most are a decent bit heavier, considering that weight doesn't even include having a stereo. Pretty much any 4 cylinder TJ you find out there (most had at least some options) will be 3300 - 3500 lbs, with the 6 cylinders around 3400 - 3700 lbs. Hard tops add weight too.