Shimano's service instructions for the Nexus 7-speed internal-gear hub assume that you will replace the entire axle unit. Taking disassembly past the point where instructions are available is all too easy. You might also want to cannibalize parts to assemble one axle that works from two or more which do not. Reader Keith Lang has provided instructions in two PDF files.

Note: this is advanced mechanical work, for skilled mechanics and inveterate tinkerers!

Shimano Nexus 7 Axle Unit Assembly Instructions

Shimano Nexus 7 Pictures.

Thanks, Keith!