I just bought a set of used 35's off of a guy for my 81 k5 blazer and he told me that with 4.56 gears 6 in lift and 35's that my axle would be too weak. what do you guys think?  

it's a GM 10 bolt by the way  

He is right. If you play hard and especailly with a locker you are playing with fire. SO do it sure but it is not best thing to do. In 91 GM increased axle size from 1.19 to 1.29 in 10 bolt and changed spline too. You can retrofit those axles in your truck for a little more strength with a diff side gear change for new spline count.  

I have had three 10 bolt rear ends 8.5 inch ring size, I ordered YUKON 30 spline axles same as stock with the Auburn mini-spool 35/12.50 16 Boggers. I've been through Detroit lockers, Eaton lockers, welds, but not the axles or the ring and pion and the spool. I have 400hp 440flbs and never broke an axle.  

I know of a 1/2 ton GM burb (not mine)with 35's that has blow rear 10 bolt axle twice with 35's and stock engine. Last time it left pieces and a oil trail on street and it was not racing, just towing.Owner is pretty thick about not upgrading it to a 9.5 and saving headaches. Myself personally before I would spend a dime of 10 bolt upgrades (like aftermarket axle shafts and such) I would swap in a 6 lug 9.5 and be done with it.

BTW, 30 spline 10 bolt axles are 1.29" dia while 28 spline (pre 89) are 1.19"  

Are you talking about blowing te rear diff out or brakeing and srtipping the axle. I've been trough many rearends but not the axles,Slung them straight out of the side of the truck picked them up cleaned them up and put them back in. I've slung the same axle out of the truck three times and has had never broke axle.  

The Auburn mini-spool is the best thing for the GM 10 bolt rearend. It keeps everything locked and tight so there is no room to brake.  

i have had 3 trucks with a 10-bolt. one was a stock blazer with a LT1, happy as a clam. the other 2 were half tons with wide 35's on them. one truck had a front 10 bolt and it went through wheel bearings, pinions and spidergears like crazy. the other had a rear 10 bolt and busted shafts and gearsets like crazy. both trucks had a queezing stock 350. front axle was open diffed, and the rear was posi, which actually is more forgiving on parts than a locker or spool due to the fact that an open diff and posi trac are made with the idea of giving a little to torque input, rather than giving all power to both wheels at all times. also the lower gears you have the weaker they actually are, fore there is less material on the gear itself (look at a 3.73 gearset versus a 4.88 and you'll get the idea). i would suggest run the 35's until you find problems, then swap in some 1 ton axles. in the long run you will spend less money than upgrading a weak axle that may or may not break  

HCRedneck:
The Auburn mini-spool is the best thing for the GM 10 bolt rearend. It keeps everything locked and tight so there is no room to brake.

This is really kind of silly because spools not only reduce direction control, increase turning radius, accelerate tire wear but they also increase strain on axle shafts big time. Even costing around a corner can wind axles up as they fight to skid tires because of different rolling radius in turns. Spools are by far the hardest on axles and open diffs the easiest and LSD somwhere in between but a lot closer to a open diff than spool in axle strain.  

i would just up grade to a 12 bolt.   thats what im running and no problems  

If you upgrade to a 12 bolt, you really want a car 12 bolt because a truck 12 bolt is weaker unlike 10 bolt car and trucks which are the same. Actually I would place a 91 on 10 bolt above a old truck 12 bolt.  

snoman

you might be ok if you don't get the truck in a bind while offroading but to play it safe change your front and rear ends out with something stronger  

If you limit it to 33's and no locker or spool a 91 on 10 bolt will hold up pretty good.