Sounds to me like drive line vibration then.
Its shows its self in all sorts of funny ways, on mine, i can feel it through the clutch pedal when changing down gears of all things
Can you check for me that you front hubs are diss-engaged properly??
Try pulling the rear prop shaft off and take it for a spin in 4x4 (FWD) and see if the vibrations still there. (Its a 10 minute job).
If your front hubs are out, that will rule out vibration front the front shaft as it will not be rotating.
By removing the rear shaft and driving on the front diff, you can rule out that one as well.
Check the left hand engine mount.
Its not a warped disc rotor is it??
My bet is mis-aligned UJ's.
Edit:
If you have the auto hubs check that the bolt in the center isn't turned to manual and they aren't permanantly locked all the time.
If they're behind hub caps how would you know?? (apart from seeing the D/S turn when you have a front wheel jacKed of the ground)
Could even with the bolt turned to auto, the hubs might not be diss-engaging properly.
If they are locked, then the vibration could well be coming from the front D/S.
Not through the D/S being out of balance, as you've had it balenced, but through the angles of the universals having a greater than 3degrees difference in angle to each other.
This can happen from coils that increase the ride height as little as 50mm.
Something else to have a quick look at anyway, its all totally irrelevant if the hubs are working properly and the front D/S is doing nothing when in 2wd.
Did you get the rear universals set up and shimmed/spaced/centered properly??
There was a case in OZ were a guy went all out trying to get rid of a vibration and after spending huge $$ found that the shop that had replaced the UJs hadn't set them up properly after all, and it was the cause of his problems.