We have a 2005 Aztek, 89k. We picked it up used, runs great, transmission shifts perfect and the torque converter locks up when its supposed to. We did a filter and fluid exchange on the transmission. The pan had minimal crud, looked good. We used an AC delco filter and dex VI which is supposed to be backward compatible. After the filter and fluid change, we  removed the lower trans line to the radiator to flush out the remaining old fluid. We started the engine for 5 seconds, the refilled the trans and did that till the fluid ran clear. Worked real well. After this, the trans developed the dreaded rev clunk or delayed engagement in first gear. After it warmed up, that went away and then it had very harsh up shifts and down shifts. We looked at the viscosity of the Dex VI compared the the Dex III which the trans was designed for and programmed for. The Dex VI is has a lower viscosity. I looked up the viscosity of the older Dex III and found Valvoline had a Dex III equivalent that had the same specs as the old Dex III.  Thought that might be a factor.  We also found a GM bulletin  that they used crummy magnets in the pan, and that the normal metal particles would cling the to the PCS solenoid because the solenoid had a greater magnetic force than the crummy pan gasket. Typical GM, save 3 cents on a critical component. The bulletin says to buy 2 new style magnets, replacing the crummy one and the second one goes directly on the bottom of the filter. We did this and used the Dex III Valvoline equivalent.  No change, drove it for 2 weeks, no change. We decided the PCS was a probable culprit. We ordered a full set of 4 solenoids and manifold pressure switch. We used Gm parts. We did the trans side cover removal, which first required the  intermediate shaft to be unbolted from the rack, ball joint removal, drive axle removal, tie rod end removed and right hand side of the front frame to be lowered. After wrestling with the correct frame height and trans height, we were able to remove the side cover of the trans to expose the Valve body. Solenoid and switch replacement was gravy. Reassembled the whole thing and filled it with the Valvoline Dex III equivalent.  Happy to say the trans shifts and performs like new. Smooth firm shifts, no delayed engagement or rev clunk at all. Cost was 35 bucks for new fluid, 25 for new side cover gasket, and 80.00 bucks for the solenoids and switch from a reputable seller on Ebay. All solenoids in sealed AC Delco bags, with AC PN's.  Hope this info helps someone!

I need to do this also to my 04. Sounds like we need to get together and help each other out!!