If you have an visions of going to 33s then do yourself a big favor now, get 4.88s your mpg and trans life will thank you!! I've driven many setups- 4.56 w/33s and 35s, 4.88s w/33s and 35s, 4.10s w/33s.....
best setup by far is still 4.88s and 33s, I don't look forward to the day I decide to jump to 35s i will miss the good power and mpg I've been getting
If colorado has the hills and mountains I think it does you surely won't regret the 4.88s. I live in NJ and we have some hills on the highway but it does amazing, the times I've taken it out to NY and PA I really got to appreciate the lower gears, not to mention 4 low is VERY low and does amazing in the rocks/trails. Not trying to bash you or anything just giving you some things to think about, I went through this expensive dilemma a year ago and glad I choose to gear it deep at the time. You may think 31s are the biggest tire you will run but trust me, it most likely wont be lol!!!!