Duramax Brake/Steering issue

We're stumped with this one - hoping someone can shed some light here...
2012 long box 2500 extended cab duramax
I drive it ALOT - 99,000km - (60,000 mi.)
It has a 6' high SpaceKap utility box on it. It is mostly empty except for a few hundred pounds of gear. Cap weighs approx 800 lbs according to manufacturer.

Truck has a little shimmy on rough roads - almost like a pitman arm or idler arm is going.
If I heat up the brakes, that shimmy becomes violent enough it feels like I'm losing a front wheel. Drive quietly for 10 minutes and stand on the brakes and everything is back to straight and smooth as it should be.
Checked rotors - they are within 10 thou of true. Turned a skin off of them anyway.
Checked temp of all four brakes right after a violent trip and all four were the same.
Checked callipers and pins everything is free and appears in good shape. Pads are all like new.
Added a factory stabilizer shock to the steering arm and it fells tighter than a new truck but problem is still there. It seems like antilock or traction control is jumping in but no codes are showing anywhere.

A secondary, presumably unrelated problem is hard downshifting under load. Local transmission shop has an Allison reader with all updates but it will not link to my truck. I'm told that GM has a bulletin that describes the downshifting problem as "normal" (???)

Booking this into the dealer is easier said than done because of location and work schedule so I thought I'd throw this up here just in case anyone has run into something similar.

Please don't tell me it needs new brakes unless you've run into exactly this issue because we've been through everything there....

.010 rotor runout is way out spec are they within .003 now? , even though any play in the steering /front bearings can be part of this. Also is this shimmy felt back thru the steering or brakes?

I would go for a 2 mile ride with a cold start drive easy on the brakes. then check the temp of the rotors.. this may be a caliper issue. with the added weight I would make sure the brake fluid is given a good bleed.

I at every pad servicing open the bleed screw then using a C clamp push back the caliper piston[s]..... this then dumps out the damaged fluid.

then you got the caliper hoses that can get damaged from the heat [internal collapse]....

This is a speed wobble felt through the steering when brakes are applied. Let off the brake and it takes a second or two to straighten itself out. There's probably a 6-8" throw in the steering wheel. Enough to shoot the entire truck into serious trauma.

Temps of callipers are constantly equal with no signs of anything burning even right after this happens. Rotors are bang on true right now.

This seems to me to be an electronic system jumping in when it shouldn't (anti-lock or traction control) but I don't know if that can be disconnected to test it.

I don't know much about the electronic systems and I'm wondering if it could have something to do with the Alison reader not being able to communicate with my truck. It doesn't seem like it should be related but like I say - I'm not up on these systems.

This is a speed wobble felt through the steering when brakes are applied. Let off the brake and it takes a second or two to straighten itself out. There's probably a 6-8" throw in the steering wheel. Enough to shoot the entire truck into serious trauma.

Temps of callipers are constantly equal with no signs of anything burning even right after this happens. Rotors are bang on true right now.

This seems to me to be an electronic system jumping in when it shouldn't (anti-lock or traction control) but I don't know if that can be disconnected to test it.

I don't know much about the electronic systems and I'm wondering if it could have something to do with the Alison reader not being able to communicate with my truck. It doesn't seem like it should be related but like I say - I'm not up on these systems.

when the ABS activates the brake pedal will pulse. sounds like you have brake pad/ rotor / grease/oil on rotors etc...

the only time I ever seen the steering wobble is with rotors damaged ...

when you cut the rotors did you wire brush the hub before install. also the rotors may not have been set into the cut machine correct ..did you do the run out with rotor on vehicle lock down rotor securely first. do the pads fit properly they are not binding ...too tight can cause problems.

If you have that much play in the steering thats what I'd concentrate on first. Check linkages,idlers,ball joints,tie rods,steering box play and mounting.Check A-frames and bushings too.

,,If you have that much play in the steering thats what I'd concentrate on first. Check linkages,idlers,ball joints,tie rods,steering box play and mounting.Check A-frames and bushings too.

could be the death wobble....this can happen at any time. usually when you hit a bump in the road the steering wheel violently vibrates along with the front wheels..

like you say this can be front end steering components damaged / worn ...