Don't think you'll need to do any gear change.. 5 th gear won't work with 29 or 31" unless you're drafting or going down hill, just slow down, cruse at about 55 or 65. 5th gear doesnt work well with mine.. 29*10.5*15 88.5 stock all around on the gears..
Unless you do some type of gearing correction kiss off 5th.
I have an 90 trans in mine, With a 4.16 GRS1 type case and 31's I could only use 5th on the flat or downhill with no wind. (Fresh engine.) With the 87 trans I never could use 5th, now with 33's and the GRS1 and 4.62 diff gears I can use 5th all the time over 40 MPH.. (I am going back to a 87 trans for the higher 5th ..) A gearing correction is really in order if you put larger tires on.
You can run 4th just fine, the problem when you go to
larger tires and don't re-gear is that you will stress your clutch and engine more. Alot of people do it but they don't get the performance out of the truck they should. If you want the meats you should correct the gearing too. I learned
along time ago to gear things right or all kind of problems will show up in time.
Yes. it will be geared down some from stock with 31's but will be drivable. There are gear calculators on izook
and off-road.com's tech section that will help with your final ratio's With just a 4.16 t-case gear set (18% lower
in high range I think) the 31's where close to right.
and the low range is kickass too. You can get them for 400.00 from a few vendors btw.. The 4.62's you need 2
front end gears from an auto tracker or sidekick. Or
new gears from Northcoast offroad or somebody. another option is an SJ410 t-case if you can find one, it is like 15% lower than a stock sami. If you luck into one you can get them cheap.. Good luck