Well, the ECU and locking motor are definitely different. Also I believe the LR3 has a 31 spline axle and the LR4 has a 29 spline axle.
http://www.4x4proyect.com/pdf/arb-parts-rd217.pdf
Its possible one could swap the LR3 non-locking rear carrier and rear hubs (31 spline) to an LR4 and then install the RD218 LR3 rear air locking kit. Assuming the hubs bolt up properly, Id be very interested in the geometry of the diff carriers to see that they're the same.
A laser scanner and software like Geomagic Design X which can take multiple STL scan files, orient, align, and analyze for mechanical similarity:
Essentially you can take two scans, or CAD and a scan, over lay them, and it will show you the mechanical differences. Red means the candidate surface is too proud or high compared to the reference, blue means the surface is too shy or low compared to the reference.
You can also just extract geometry from the mesh and then compare coordinates with measuring, like so:
Also, here's some more shots of the locking rear, after a small amount of cleaning the rat ******** and piss off of it:
![UNADJUSTEDNONRAW_thumb_1380.jpg UNADJUSTEDNONRAW_thumb_1380.jpg](https://expeditionportal.com/forum/data/attachments/351/351140-f640114b120abced52ba6aacbaf1c6f6.jpg)