I had this issue, I guess I can't say for 110% sure it was the 3-4 clutch pack, but it probably was the 3-4 clutch pack. I didn't try the solenoids, but you can. At 258k miles, you may as well rebuild it, in my opinion. I started having troubles and tried chipping away at it, I learned a lot but in the end it would have been faster and easier and cheaper to just go ahead and do the rebuild. If you fix the just solenoids now, in 6 months the 3-4 clutch pack goes anyway.
Are you saying you would do a rebuild now and then sometime in the future do an improvement kit? If you are, I wouldn't do that, I would get it all done at once, the first time.
Really the only thing I asked the rebuilder was if he would do the upgraded sunshell. Otherwise I just went with what was he suggested. It's not like I was going to be able to check his work.
Cool kit is cool, but with the 4.2 in an '06, are you racing? I rebuilt the lower myself before the whole thing got rebuilt, I did upgraded pistons and separator plate. It was an interesting learning experience, but the only people I beat in a race off the line are the ones that don't know we're racing.
On the other hand, it's fun to optimize and improve.
The lower is pretty easy to get to if you just want to drop the pan and start poking around, and that's where the solenoids are accessible.