Check Engine & Trans fluid questions

[jersey]

Sad Update:

Did the Thermostat as recommended, however a few weeks later the plastic radiator cracked open. Had that replaced and did the tensioner too when it was pointed out as the source of the noise I was hearing in the front. In addition to this had the fuel injectors cleaned and the engine tuned. Left happy, but still didn't get the trans fluid changed. Next thing you know the car started with a shaking at speeds of about 40 MPH. Nothing serious at first, but made an appointment at the trans shop anyway. On the way there the trans popped. Case cracked. No surprise the shaking stopped as I rolled to the side of the road. Onstar towed it the rest of the way to the shop. Replaced trans, but shaking continued. It was determined the Universal Joint was slightly warped and that's likely what caused the shaking and probably what caused the trans case to crack in the first place. Had that replaced about 10 days ago and now the shaking is slowly coming back. I immediately brought it back, but my mechanic (in the biz for 20+ years) checked his work and says it's not the universal causing the problem this time and so he pulled the shaft and it seems to be running true too. So far he can't identify whats causing the shake. Just a little FYI he worked as a dealer mechanic for Hummer through 2009 and now operates his own shop.
If any one has any suggestions I can pass onto him, I'd appreciate it.

OOOP's: Forgot to mention that the rear leaf springs were changed with OEM springs (#25965051) in September 2012.

Question: What's the stock height for the rear of an H3 from 2006? Maybe the springs are defective in so some way and thats affecting the driveline somehow?