10 bolt limited slip to open

[Nad_Yvalhosert]

Yup, that looks like a width difference, likely a 2wd 14 bolt. Oops! Just throw on a set of bolt-on spacers, say 2", and drive it.
You sure the 14 bolt came from from a 400 truck? Not a Square body or Express van?
Hopefully you checked the gear ratios between the old and new rear axles? Mismatched axles will be a real problem the first time you get fiesty...
Ask me what 3.73 in the front and 3.42 in the back was like (for just one time in the snow until I could send the front out for rebuild)

 

[Schurkey]

SOMETHING is screwed-up. Likely the seller's description of the axle(s).

A 4WD GMT400 14-bolt semi-float 9.5" axle in place of a 4WD GMT400 10-bolt is a very common, direct-bolt-in swap using a 447 conversion U-joint. Even the GMT400 4WD Suburban axles will work although they have mounts for a sway-bar you wouldn't need. If you're having problems with the U-joint AND the width...I bet that's NOT a GMT400 4WD 14-bolt semi-float axle.

 

[RichLo]

Agreed with above, something wasn't advertised right.

And you can throw on some 2" wheel spacers and be fine. Just Please get the good brand name spacers, not the cheapest Amazon specials! There is a huge difference in quality! Believe me...

Aluminum wheel spacer fail on 14bff

This just happened on my plow truck. Luckily I was just doing some drift cleanup and nothing too important and I caught it before the whole thing came off. This spacer was on there for 1 season last year. It looks like it was wearing down for a while. It looks like the cheap lugnuts they...

www.gmt400.com

And re-torque them after some driving just to make sure everything is seated nicely.