Change ratio in rear diff = change ratio in front diff?

[RINNY]

2003 astro AWD with 3.42 in rear diff want to go to 3.7 or 4.10. If I change the ratio in the rear differential do I have to change it in the front differential?

[Herbie]

2003 astro AWD with 3.42 in rear diff want to go to 3.7 or 4.10. If I change the ratio in the rear differential do I have to change it in the front differential?

Yes. You need your wheels to turn at the same speed (more or less). Any difference will result in one set of driven wheels trying to go faster than the others, which would mean either something needs to slip, or you'll break gears. Going around corners causes some variation in speed, absorbed partially by the diffs (left to right) and tires (slip). Very small differences (like down to inflation diameters, etc.) are also absorbed by the slip in the tires, but anything as significant as a .3 or more difference in axle ratio would cause massive problems.

EDIT: The above applies mostly in a locked center situation (true 4x4 transfer case). The AWD transfer cases can absorb a slightly larger difference between front/rear axle speeds by slipping in the center instead of at the wheels, but even that only accepts VERY small changes (again, cornering), not a numerically different axle ratio. Trying that with an AWD TC will burn up the TC. Trying it in a locked 4WD TC would probably break gears unless the surface was very slippery and one or more wheels were basically sliding along...

[RINNY]

I Kind of figured that so the next question is how difficult is it to regear the front differential? Should I just get a front one geared the same? any other problems such as reprograming and such?

[Captn. Crunch]

[quote][/quote]
As Herbie said, you can't run markedly different ratios. We do have a member running 4.10 / 4.11 setup but he has 4x4 not AWD and that small of a difference is easily nullified by loose footing off-road. Our early AWD vans utilized a viscous coupler transfer case that would fail if your tire pressure was not consistent on all four corners causing different circumferences.
You could re-gear your front diff or you could replace it entirely. You will need to pull the diff out to re-gear, open it up, tear it apart, put it back together and re-install. Search online for a diff for our vans with the 3.73's. 3.73 ratio was also used in all S-10 ZR2's so any late model ( with slip in vs flange style axle) will bolt right up. If your lucky ( I was!) you might even find a cast iron diff that only came in a very select version of truck. 4x4 manual trans certain t/c's.
Now all you would have to do is open it up to verify ratio, check bearing wear and such. Slam in some fresh seals and good to go. You can leave the two part S-10 axle and install a cable actuator if you decide to change out t/c's and gain low range. Or you could just as easily swap your one piece axle over for back to original setup. No issues with reprogramming unless you change tire size and want your speedo to read correctly