hi, i have a 1981 full size jimmy, and i want to run 37 inch tires, and dont wanna switch out whole new axles.

i was wondering if just upgrading the axle shafts in front axle/ both axles would be safe for most offroading situations. im not into rock crawling at all, so it wont be too extreme, mostly trails and mud.  

You actually could get by with the 1/2 ton axles if you drive very carefull.  35" tires are really the safe limit even with alloy axle shafts.  But driving style will be more of a determining factor of breaking or not; so going by what you described you will most likely get by with 37".  To help the front axle assembly live I would recomend you leave it an open differential; go ahead and put a locker in the rear.  You will need to change the gear ratio.  The origional gears are most likely 3.07:1 and the front 10 bolt you will just need the gears (and very likely bearings and shims); the rear axle may be a 10 bolt or a 12 bolt.  I am not sure which one it would have come with.  I know 1980 had a 12 bolt and 1982 had a 10 bolt.  If it is a 10 bolt the required parts (except for locker) will be the same items as the front (different part numbers though).  If it is a 12 bolt you will need a carrier for 3.73 or higher numerical.  I would recomend no taller than 4.56 gears, with 4.88 being a better choice.  

its a 12 bolt rear, and 10 front. factory 2.73... flies on the highway.
i have 4 spd w/ low so gears right now i can get away with cause i got a 46.7 crawl ratio with factory 235/75's, but i gotta factor in the tire size being bigger. im just doing a little off roading right now, i cant push it, cause im in school and cant afford to break anything too badly. later on im going to buy my uncle's old flat bed 4x4 with a dana 60 front and 4.56 gears... but right now im poor.  

I run 35" tires on my 96 GMC with IFS and i run it pretty hard in the mud and in the hills. And i havent had any problems with anything in my axles. I have limited slip in the rear with 3.73. the only problem with it happened on pavement and my rear end went out when a carrier bearing came out and tore everything up. But if you are going to be going with 37's you should prolly change the gearing to 3.73 or 4.56.. And i would bet you could get away with the 37's without putting in new alloy axles.  

If u are easy on the axles they will live for awhile with 37s, but any aggresivenes will kill them as i broke a rear 12 bolt with only 33s but again im agressive driver and have big motor with locker in back so after breaking i went to 14 bolt ff 1 ton axle.  You will need to run at least 410 gears with 456 gears being just right and 488 being too deep.  

oh really? this blazer i drive by every day is running 39.5" boggers and hes got stock 6 lug axles.... hes always covered in mud. i wouldnt mind knowing what secret hes got to keepin that baby alive  

i did some math, and my truck in low gear with factory 2.73's, has the equivalent crawl ratio of it in 1st gear, with 4.88's... so if i treat low gear as 1st, 1st gear as 2nd, and so on, i should be okay until i can afford new axles. ill just have to put up with the lack of synchros in low.  

As far as the 39.5" on 1/2 ton axles; Like I stated before, driving style plays a big part if something breaks or not.  I have seen 44's on 1/2 ton axles.  When driven very hard (like tuff truck competion) 33" tires can push 1/2 ton axles beyond their limit.  

I'm quite sure your gears are either 3:08, 3:15 or 3:73, maybe 3:42 or even 4:10.
I have never seen a blazer higher than 3:08 and I've owned about 10 of them. check the tag on the front, if it's still there.

this is the only one I have that's not lifted. 75' with 3:08's on these 31's it does about 130mph and with the t-case in low range it does almost 60mph, I'd really hate to have 2:73's  (turbocharged small block)


 

That is a very nice, clean looking Blazer mudb8.  The full time GM trucks had slightly different ratios front and rear.  Example: 3.08 in the rear had 3.07 in the front; and 3.73 in the rear had 3.72 in the front; and 4.10 in the rear had 4.09 in the front.  If you pull both covers and check the numbers stamped in the ring gear, you will find this to be true.  I have 3 full time GM trucks (78 Blazer, 76 1/2ton pickup, and 76 3/4ton pickup) and they are all this way, I have checked many other full time GM trucks and all are that way that still have the factory gearing.  

thanks,
yep, if it has a 203 case it has the high/low axle ratio, I have a few also.  

hmm... well the chump who i bought it from said 2.73 right from the factory. i would check, except its getting a full body resto done right now, not getting it back till around december 20th. maybe the first gear is the highest possible for the SM465 (2.47?), thats why it seems like 2.73's, but isnt.  

I'm frequently wrong about some things but i doubt it on that one.  

There are two other ways to check your axle ratio without pulling the differential covers.  If it has not been removed or torn off, there should be a small metal tage on one of the lower cover bolts with tooth count numbers on it.  Another way it to mark the tire and drive shaft, either drive or roll the truck and count the drive shaft revolutions for one tire revolution.  Personally I have never come across a truck with 2.73 gears.  I stand by my original statement that it should be 3.07Front/3.08Rear.  It would be interesting to find out for sure after you check it.