1st gen Diff swap
 

Recently had the steady bearing let go on me down the highway, unfortunately it took out the rear drive shaft, T case and did a bit of damage to my rear diff (carrier, spider gears, bearings). I've located a Np205 29 spline and sleeve to replace the old one. No luck getting any new parts for the rear Dana 70 4:11 in Alberta. I have tracked down some axles from a W250 SRWI don't have much experience with diff is there any issues swapping the internals of a SRW rear to my Dually?

My truck is a 92' W350 4:11 with the Getrag
thanks

The center section parts should interchange fine...
That said, 1st gen Dana 70 axles come with different spline counts on the axle shafts. this would be the largest potential pitfall of your plan.

Earlier ones ['89 maybe '90 usually I find] have 35 splines then the later ones seem to usually have 32 spline Dana 70 axle shafts.