How much wider - dont know the exact measurement before, but I am 82"(ish) front outside of tire to outside of tire.
Why did I - because I trashed my stock d30/amc20 and got a killer deal from a friend on complete axles.
Am I glad I did it - oh yes. Fullwidth is the way to go. So much more stability.
I swapped in a rear D44 from a Scout--very straightforward. Also swapped in a front D44 from a Wagoneer, and had to do more fabbing and ran into way more issues. Went from narrow trac CJ to about 58", so I gained quite a bit. I wanted more stability and strength, and I'm glad I did it.
put on Currie 9" axles front and rear, went with TJ witdh from WMS to WMS (61 1/2"). Wider than stock (52"), gives the Jeep more stability and a much more agressive look, but retains one of things that makes a Jeep better than a fullsize to wheel... it's size. Expensive to go the custom axel route? Yes, but I got brand new assemblies with the gears and lockers of my choice, shipped right to my front door and bolted right onto the Jeep with no major issues.
I went ford 9 inch out of early bronco in the back, stock widetrack front with disc conversion from '85. It was narrowtrack, I like it better, more stable, stronger and easier to get parts for than the jeep. At least where I live as parts go anyway.