2003 G35 4Dr sedan auto 3.5l open diff, swap out rear sub-frame and diff from an 03 350z auto with locking diff, is ther any fitment issues?

 

Is the 350z stock diff higher than 60k miles? If so I wouldn’t recommend it, as the VLSD May have likely wore out and won’t do its job.

 

It would be a lot less work, a lot better performance, but also a little more expensive is to switch to a good quiet clutch LSD if you're familliar with tearing down a differential. The stock VLSD is pretty crap anyways, it's better than NOTHING but the engagement time is slow because of the inherent reactive nature of a VLSD. A clutch LSD is very proactive and also works when decelerating not just acceleration, and a good one like the ATS Super Silent is worlds better than the stock unit, Nismo one is great too.

As for your question, the axle will fit, not exactly sure why you want to swap the rear sub frame member though, it's the same on both vehicles I think.

 

Would it be better to just weld the open diff? I know it sucks on the street but heard good things with drifting it.

 

It's popular because it's cheap. Doesn't mean it's good though

For a drift car that won't ever get driven on the street then sure, weld it up.