Vibration

Alright, here's the problem: I have a vibration between 35-65 MPH. I've replaced the tires (they needed it anyhow), balanced them twice, replaced U-joints, had rotors turned, and replaced the brakes. Searched the forum also and only found two more suggestions outside of having the tranny rebuilt. 1) Replace driveshaft. I looked at the driveshaft, and it didn't appear bent, and both weights were still in place. Still, this could be the problem. 2) Transmission mount. I'm going to check this and see if that is what's causing it.

Just wanted to find out if there's anything else I should check, or easy service items that could be done to narrow down the cause.

Another piece of information that may be important...I have a pair of helper springs on that raise the back end about 1/2-3/4". This changes the angle of the driveshaft to what appears to be MAYBE 1-2 degrees. Don't know if this could be the cause, but the vibration didn't appear until about 4-5 months after I put them on, so I don't think it is.

ANY help is really appreciated at this point...losing my marbles trying to figure this one out.
99 Envoy, 106K miles, 4x4.

Also, as an update. I was tooling around underneath of it tonight and found something that I'm not sure is supposed to be happening. With both rear wheels off the ground, tranny in neutral, when turning the rear driveshaft the front driveshaft turns. Vice versa, when turning the front shaft, the rear shaft turns.

I had thought when in 2HI the front is disengaged from the rear. Am I wrong, or do I possibly have some serious issues with the transfer case?