Saying that 31's and 3.27's would be undrivable is just plain wrong. It's not ideal, but in reality it's just fine.
A few years back, I put on a lift and a set of 35's. I ran those 35's with 3.27's for over a year. Yes, I could only use 4th on the highway. Yes, it wasn't great. My mileage dropped VERY little. I was still averaging over 20 on the highway. As MAKG already said, it's all about how you shift.
Stock tire size should be 235/75R15, which according to the calculator is 28.9" tall. By going to a 31, you're changing by about 6.5%. That's almost negligible. Do the tire swap, keep the gears, and change them whenever your budget allows.