Will a rear end out of a 04 dully fit a 01 dully?

Yes. You will have to modify a few things though. Yoke is different, drive-shaft will need shortened, you will have to put 17" wheels on, 16"s wont fit with the new brakes on the AAM axles. Lets see... Rear speed sensor might be different, I don't know. Traction bar might need flipped over to get it to fit, I don't know. Why are you wanting to do this?

The leaf spring saddle needs to be cut off and moved as well as shock mounts. AAM gears came as 3.73. or 4.10 Dana was 3.55 and 4.10 options. 3rd gens do not come with a sway bar option like the second gens did.

The leaf spring saddle needs to be cut off and moved as well as shock mounts. AAM gears came as 3.73. or 4.10 Dana was 3.55 and 4.10 options. 3rd gens do not come with a sway bar option like the second gens did.

Yeah see I knew I didn't know. Basically I was getting at that it would be a lot of work compared to finding a similar replacement.