A few months ago, I noticed my truck slight "shuddering" when I hit the speeds of 40-60mph. We chalked it up to maybe it being my AC, as we had hit an extremely hot spell, and my AC seemed to be struggling to keep up with the hot weather and cooling the vehicle those few days. My dad's friend told me to test it by turning off the AC, and see if it still did the shuddering. I noticed it that it did, but it was so slight, I just kind of brushed it off. Now 2+ months later, it has gotten extremely worse, and I have not been using the AC at all. It happens like clockwork when I am accelerating in the speeds between 40-60 and it happens a few times. When I let off the gas, it doesnt seem to happen. The other day, my husband had me put it into 3rd from Drive, and we noticed that there there was no shuddering. He thinks that the overdrive may be going out....I'm not sure what all that means, or if there is something else. Anyone have any clues as to what is going on, or any ideas to throw out? I'm at my wits end....

 

Sounds like torque converter shudder. The 2003 Trailblazer doesn't have a MAF sensor which is typically the cause of most lock/unlock problems in the torque converter (along with a host of other issues).

Have you tried cleaning your throttle body? The electric throttle bodies used on these trucks can get gummed up and sticky after a while and need periodic cleaning.

Beyond that, you may have to get a full feature scan tool onto it to monitor the output from the accelerator pedal position sensor, throttle actuator, etc to see if there is anything dropping out dramatically that could cause the torque converter clutch to lock/unlock rapidly.

 

No, I haven't tried cleaning the throttle body, but I will!
I'm trying to eliminate all the "possibles" before having to bring it in for a major overhaul...but thats what it sounds like might inevitably happen. *sigh*
It sounds like if it's the torque converter, I'm in trouble....booooo!

 

You may or may not be in trouble if it's torque converter shudder. I had that problem with my 3/4 ton GMC van and a good, thorough transmission flush and filter change fixed the problem for me.

Torque converter shudder happens when the torque converter clutch tries to engage but doesn't hold. It's generally caused by old fluid that's losing it's friction properties. I'd take your truck to a trusted transmission shop and get their opinion.