Need ideas. 15yr old vibration that just won't go away

[Tom]

I bought my 98 back in 2006 with 75k miles on it. Had a slight shudder when the torque convertor stalled [1400rpm ish] when accelerating from a stop. Since then I've:
-cut and balanced driveshaft
-new u joints
-replaced entire rear axle
-rebuilt transmission
-new torque convertor
-replaced all engine and trans mounts
-inspected flexplate for cracks while trans was out
-multiple sets of tires and different set of rims
-harmonic balancer

Nothing has stopped it. It got much worse after the trans swap from flexing the motor mounts. Just finished changing them, and the vibration is back to when I bought it level of annoying. WTF??? What else can I check???
Vibration isn't there once the truck is moving. Its only when starting off from a stop. It isn't there once the rpm's are over 1500 in 1st either. Its almost like running over a rumble strip. I'm lost.