I have an 09 Trailblazer 330 that I just bought used. We have found that while riding it, many times on bumps it pops out of drive and back into neutral. The dealership said they would have to take the tranny off to figure it out. I believe it's not internal. We put a bungy cord on it to hold it forward, and we can ride it with absolutely no problems.

 

Dealer in all likely hood is telling you the truth. What you're doing by having pressure on the shift lever is also causing more pressure on the shift fork and engagement dog gear(6) to stay locked into the drive gear(4) lugs. Sounds like the lugs are worn and not staying locked up.You can try to adjust the shifter linkage to where neutral is a tad bit lower than center of the rubber slot. This may allow more forward travel room on the shift lever allowing it to stay engaged. If it doesn't help,then splitting the gear case sounds like what needs to be done. Another wear item is the silent chain. It can cause the same problems of grinding,jumping out of gear also. They always need to be replaced along with any worn gears,shift fork if worn and the oil seals.

 

Thanks for the information, I appreciate it.