[steve500]

I'm betting cross contamination of liquids... Engine coolant made it's way into the coolant lines of the transmission. Radiator has a built in transmission cooler that is suppose to keep the transmission fluid and engine coolant in separate systems but is prone to rupturing and causing serious damage to the transmission. If it happened SUDDENLY and you didn't feel any strange vibrations or odd shifting habits before this happened, I'm counting on your ECU to be toast, bad ground somewhere probably, heck even a bad battery/terminals. My ECU's ECM(EDIT, TCM*) went crap on me and my transmission only shifted up to 3rd gear, past 45mph it would slip to neutral. New ECU and shazam, perfect, found a bad ground on the engine that was ignition related that caused the problem.

 

[steve500]

I'm going to count on the coolant/trans fluid cross contamination being the culprit.

If there is a pinkish milkshake appearance under the radiator cap at all, you've got issues ! If they can, pressure them into throwing a new transmission in there and new radiator and you're good to go. I've read a couple stories on this thread that some people have just flushed the transmission a couple times and its back to perfect with a new radiator.

The ECU has its own built in TCM [trans control] (made a typo in the previous post), it is the brain for the transmission also. So, yes... when my ECU took a crap it happened all at once instantly, after it went stupid, test driving it around, it would jerk sometimes, and only sometimes... but never shifted above 3rd gear, bad grounds fry sensitive electronics!

 

[steve500]

The transmission has a valve body in there that controls the shifting components, but as far as I know, the engine computer (ECU) controls the shift patterns, this is how the truck does it's automatically adjusting shift patterns depending on the drivers style of driving for saving fuel or more more torque, pretty sure that's all in the ECU... All I can say is, when my ECU went retarded because of the bad ground on my engine (called the ignition system ground), it stopped communicating with the transmission correctly and if I tried to get it to go highway speeds after this by feathering the throttle, it was kicking in and out of incorrect gears and yes, it hit HARD... i mean it was a loud bang, to this day I am VERY surprised my rear differential is working perfectly after that abuse it took when my transmission/ecm went haywire, locker and all still works perfectly smoothly. As soon as that new ECU was put in there, the truck was perfect, oh.. and the new grounds so it didn't happen again, I made them throw me in a transmission, they told me it needed a new transmission, so they ordered one... I told them in the mean time of waiting for the transmission, throw a new ECU in there... it worked perfect, nice smooth shifts, full of power, brand new, had them put the new transmission in anyways. It was just a bad little situation for my truck, doesn't happen often, little stupid bad ground and the dealership was just lost and didn't know what to do, I felt like I did more thinking work than they did.

Just have them check simple things first, Reset the ECU, clean the battery terminals, try a different battery, look at all the major grounds (left of the battery bolted to the body, to the right of the air box bolted to the body, the top of the engine, right side towards the front of the truck, near the alternator on the engine block, etc)...

When my rodeo had a bad ground cable that I hadn't realized i screwed up on the battery after I installed my power inverter for using tools at work, that transmission went stupid also, same ignition system with coil/plug, same ECM/TCM setup, it kept shifting hard only randomly, when I went to accelerate.. it was just .. BOOOOOM! .. scared the hell out of me... after a while of tinkering, seem my ground was so loose it was barely touching, bolted er' back up, it's been 70k miles since and she shifts perfectly smooth.