My Avalanche, when going appx 65-70 miles an hour rocks violently from side to side. If you go over a bumpy bridge at this speed, it will litterly hurt your neck and you have to hit the breaks immediatly to bring the speed down and stop this violent shaking. Literally feels the like the body is going to rock off the frame!

that your stabli-trac kicking in ----check springs/coils

Very good possibility that you have a tire with a belt that slipped.

most use vacuum to hold the throttle open.. some use a signal off of the speedometer sensor..this sensor may be going bad..

LIMP IN MODE re-set your pcm if it re-occurs you may have a transmission slippage issue or a faulty shift selonoid in transmission. with stabilty control feature on some models a bad wheel speed sensor or hub bearing can cause the same but usually accompanied with ABS light

Actually they can seperate/come apart. when this happens the cv joint will flop around untill it gets to the point it locks up your tire. You can imagine your wheel locking up on the freeway going 70+ miles an hour. Ill draw you a little picture:


They can and WILL seperate locking up the tire as it flops around violently, eventually finding a spot that won't allow it to flop around anylonger. The flopping around at highway speed is enough to total out any vehicle not to mention the locking up of the wheel.

You might jack up the truck on the side, leaving one front wheel on the ground, the other suspended. Have someone watch the steering components close to the suspended wheel while another moves the steering back and forth at a medium rate and look for 'unnatural' movements of joints. Do both sides to check.
This sounds like a tie rod end that is shot and these are fairly easy to spot since the joint will move in directions that are obviously wrong.
The reason it doesn't want to stop is that when one wheel is no longer being held rigidly, it will 'oscillate' causing the violent shaking. The boost from power steering worsens the problem. It can be dangerous; a friend had a worn tie rod end come apart while barreling down the Freeway. Steering with one wheel doesn't work. He survived but no longer ignores the signs.
If you had brought the truck to a decent front-end alignment shop, they would have found this while trying (it doesn't work) to align it.

check the drive shaft joints for worn/siezing

that is usually caused by bad engine mount this condition usually changes the distance of the cv joint and the differential which cause the cv joint to shake. also check your ball joints.

is the damper in good order seems odd the tyre is worn uneven check shockie and or wheel alighnment

Ok you have a 4 cilynder engine, and you transmission is bad, and you are forcind the engine because you need more RPM that the normaly need it for certain speed, don't continue ussing you car like that because you will brake you engine too, and you will spend depend where do you live

a use transmission around $ 7 or $800.00 Rebuilt around $ 1200.00