So long as the "male part"/splined section of the axle goes into the "female part" fully
(ever how deep it is)
I would not sweat it!. A 9" is plenty strong to begin with......
How much hoss-power you planning on putting through it?!
2&1/4"  off each end shouldn't be a cause of worry.
6sal6
PS..  go ahead Don!!!....say it!!

If the shafts are the same diameter on the splined section and unsplined section you could have them cut down and resplined.  The issue you get with that is that the new splines are going to be weaker because you are going to lose the surface treatment when this is done.  Factory axles have a very shallow heat treat anyway.

If you're doing this just for the look of wider tires it'll probably be fine.  If you're doing this thinking you're going to put some sticky meats under it and get the thing to hook up some real power do yourself two favors:

1.) get custom axles made.
2.) get rid of the crush sleeve and go to a solid spacer.

Most 9" are not as strong as a lot of people think.  On the street you'll likely never hurt one.  But racing is a different animal.  Once traction enters the picture stuff that seemed fine starts to fail pretty quick.  The factory 9" cases that are not nodular are not terribly strong.  The pinion bearings are too close together, and the crush sleeve oscillates under load.  The pinion gets slightly cocked and breaks the case right at the pinion pilot bearing bore.  I've seen it a half dozen times.  The solid spacer will typically solve the issue in most street/strip cars and 4x4s running reasonably sized tires.  The next weak link is the cross pin in the stock diff. 

Its all about where the fuse is.  With a street car the fuse is almost always the tires.  Most times, in 1st and 2nd gear at least, the tires will just spin before anything can be shock loaded enough to cause a failure.  3rd is where you need to be a bit cautious, because most cars under 450HP will have traction in 3rd, even on street tires and that's where things can get shock loaded.  Normally though you see transmission failures from that, not rear end failures.  Rear end failures tend to happen at the line on a hard launch.  I think you'll be okay. 

Better to make the improvements before that happens.  If something breaks at the very least you're going to ruin the bearings in the rear, possibly the gearset as well.  Cheaper in the long run. 

Thanks bob...I’ll be sure to check.

Yup, new Axels  are in my future. Got the diff all stripped and ready to be narrowed.