What will that do to my freeway speed? I ask because I do not trailer my rig. I drive it. Sometimes up to 250 miles. With lowering my overall gear ratio, will that be bad for freeway driving? My friends are telling me to just stick with the 4.625's and lock the rear, and I will be fine.

You are running 33s right?  If so, you should be fine with 5.12s.

No, I am running 31's. Will that make a difference?

You'll prefer the 5.125s.

Well, your looking at about a 10% drop.  So if you turn 3K at 60 now, you will be turning 3.3K instead.

I've got 5.12's and 31" tires, if you drive on the freeway you'll want to use the right shoulder.

According to my calculations, if you are running true 31s (which you are probably not....so your RPMs will be slightly higher at any given speed), and you have the 3 speed auto tranny (1:1 high gear), then your RPMs will be as follows.......

MPH         RPM with 4.62s         RPM with 5.12s
60               3008                        3333
65               3258                        3611
70               3509                        3888


These numbers also do not take into account any slippage in the tranny (which with the torque converter locked up should be none or very minimal), but if you are getting any slippage, the RPMs will be slightly higher.

Err ha. So basically, I can run those gears, but i will have to add an additional 45 mins to my drive time to get anywhere. (ha)

Your drive time will be the same, you will just run at higher RPM's.  Of course, if you are already running larger then stock tires, this will just bring you back to where you where before.  Or closer to it. 

Err ha. So basically, I can run those gears, but i will have to add an additional 45 mins to my drive time to get anywhere. (ha)

3900 RPM at 70 isn't that bad.  You'll probably be a little higher than that, since, like I said, your tires probably don't measure an actual 31".
I was trying to think what one of my 4 door trackicks run for RPMs at 70.....I was thinking around 3600.....so you'll still be in the "ballpark" and you will probably want to run a little higher RPM than "stock" because those tires will be a little harder to keep turning (because of the additional weight) than the lighter stock sized tires.

Oh, and these engines don't redline until 6500 RPM.....so you should be ok running 4K-4500 RPM for extended periods.  Just keep the maintenance up on the engine (like you should on any engine, no matter whether you are running it at higher than "normal" RPMs or not).  Keep the valves adjusted, the oil changed, etc.