how strong are they?
 

I have a 79 xj6 s2.
How strong are the rear ends and axles in these things?

I'm planning on swapping in a mildly built LS. Possibly turbod.
Wasn't sure how well they held up to power.
Or if regearing them is cheap and easy....

Kinda depends on your definition of mild.

They survive the 320ish stock hp of a 6.0 V12 XJ-S. I think the weakest point is actually the rubber mounts that hold the cage to the body. High power, particularly applied violently such as in a drag race, can rip them. I think people in that situation tie the bottom of the cage into the body.

Well, I currently have a 700hp LS jeep comanche pickup, so mild is probably 500ish lol

What are you calling the "cage" on these? I'm not familiar at all with anything jaguar lol

The differential and rear suspension all attach to the "cage".
https://cimg7.ibsrv.net/gimg/www.jag...0b474173e8.jpg

It's mounted to the car with two rubber mounts on each side.
https://cimg5.ibsrv.net/gimg/www.jag...ce12f633d9.jpg

The cage holds the diff, the rear brakes and calipers, axle shafts, wheel hubs, and suspension.
https://cimg9.ibsrv.net/gimg/www.jag...57db5f09e0.jpg

My car came with a 350 SBC from a 1974 Corvette. Even that was enough to damage the cage but I don't know how hard of a life the car had when it came to me.
https://cimg3.ibsrv.net/gimg/www.jag...b9498d4309.jpg

It's essentially a Dana 44 rear differential, used in damn near everything. DDG it...

So are they're any upgrades for the axles, diff, cage components?
Brakes?
I assume there is for shocks and stuff

I've got a few thousand miles on my swap so I figured I'd chime in.

The rear end strength (and really any drivetrain component) is dependent on how it is treated. You could probably break the IRS with a warmed over 350 SBC if you dumped the clutch repeatedly with drag radials on a sticky drag strip surface.

In contrast, my 1984 XJ6 has a nearly stock rear end with the exception of 3.54 gears and an Auburn Posi. It has survived a year and a half of nearly daily beatings of estimated 500+ ft-lbs of turbocharged LS torque (10-12 lbs of boost, E85, stock Gen 3 6.0 with SS2 cam). I attribute this to minimal shock loading, which is softened by a 3200 stall converter and relatively narrow 245 section width tires. The turbo helps cushion the torque hit, unlike a strong built NA or supercharged LS which can deliver all the torque, all at once. My experience is 100% on the street, so I can't speak to how it will hold up on a drag strip. If the tires are the fuse, it's hard to kill the differential/axles.

My differential slowly started to make noise this past fall cruising at 30 mph, due to a lack of maintenance over its 174K life - no lubrication galled up the cross pin in the open diff, which came loose and started to whack the pinion gear. Nothing to do with excessive torque.

The only "upgrade" that was absolutely necessary for the additional torque was the addition of bars that connect the bottom of the differential subframe (cage) to the unibody where the center support bearing used to mount. Without them, the front of the cage rocks upwards and your driveshaft will rub the tunnel badly.

All of this is documented in detail in my build thread on the forum (search under my username).

Everything he just said!!! My engine is a warmed over gen2 lt1 and I put the auburn lsd with 3:73 gears, also used that time to upgrade the brakes! But yeah the cage twist is a real thing!!!