I don't know. Even the new F-250 pickups (or at least the F150's) have independent front suspension with CV's and axle shafts on the 4wd ones. Not to mention all the toyotas and stuff since 1985. They're a lot more beefy than a subaru axle, but similar design. Wonder how easy it would be to take the IFS from a Tacoma and put it in a subaru with a diverced T-case?
You can break even solid axles if you throw enough torque at them.... I know a guy with a heavily modded cummins turbo diesel in a lifted 4x4 E350 van. He was running 44" highway rated excavator tires on it for a while, but kept grenading rear ends and driveshafts and such, so he went back to 39" or 41" truck tires.... I was following him down a jeep trail in my stock '82 GL wagon one day... sure I had to pick my line carefully and was taking some undercarriage and rocker panel hits... but I kept up fairly well considering
but once again... you go back to the cost to strength ratio... how much will it cost for you to buy a toyota or f-250 just to remove it's front end? it'd have to be a tacoma or later... you can pick an 86 to 95 wide track toyota solid axle for a hundred bucks, springs for 50 then say another 50 for the rest of the parts... so 200 dollars versus 5000 to 10000 dollars to part out a better offroad vehicle... hmmm... lets think about this... if you are already going to a non-subaru part, you might as well get a part that will perform while acutally 4wheeling.
not to mention they are 4.10 gears in the yota 8" axles... so if your using legacy diffs, you don't even need to swap out new gears. just weld the diff and go.
early toyota front ends are also known as garbage, they are fairly decent for offroad racing but lock them up and they tend to start breaking. and yes if you don't build up your axles to match your cummins and 44's your asking for trouble. rock buggy's swap in dana 60's but they go with heavy duty axle shafts and heavy duty drive shafts. you can break anything if you don't build it right.