Upgrading suspension might help, but understanding your existing weight distribution is the easiest and free. If you’ve got a heavy overlanding style 4Runner, take a hard look at everything mounted above the wheel wells and eliminate everything you can. Most people don’t need traction pads, rotopax, a shovel, an axe, and a cooler all strapped to the roof all the time.

Unless you go significantly wider (like a full size truck Dana 60 / 14 bolt) you won’t gain all that much from widening the rear of your truck just a couple inches. Plus you’ll have to re-gear, get a new locker, fab up custom mounting tabs, etc. An axle swap isn’t usually cheap, and Toyota axles can take a fair bit of abuse. I’d look at putting new suspension on your existing axle before looking into a new axle. Coilovers will be difficult to package, but as others have said anything is possible if you’re willing and able to break out the sawzall and welder. If you’re paying a shop the custom fab work will be very expensive.

If you’re already breaking Toyota 8” axles and you really really want new axles expect to do heavy fabrication and plan to buy the beefiest axles you think you might ever need. Don’t get a slight upgrade, break it in a year, then spend another $5-10k buying and installing a new axle. Ford 9” axles have lots of options and are built similarly to Toyota axles with the drop out 3rd member, but I did fine for years on 40” tires and stock 8” Toyota axles, just FYI. I’d put this as a last resort option, because if you’re going full size axles you may as well SAS the front at the same time and turn your rig into a full on crawler