I have a 2000 4x4 Excursion with a regular, pulsing vibration that occurs at 20-30, 45-55, 65+ mph. The vibration comes and goes every 3 to 4 seconds depending on speed. It is worse under load, but doesn't seem to be particularly torque sensitive.
The truck has a 3.5" Old Man Emu lift, brand new Nitro Charger shocks. The brakes and suspension are all new -- everything below the frame with the exception of the gears and rear bearings are new parts, properly installed, thoroughly checked and re-checked. The driveshaft was balanced with new u-joints and the yoke runout was checked and was within tolerance -- this driveshaft is not a double-cardan, it uses a single u-joint front and back. The tires are 35" BFGs, road force balanced. The wheels are new and were checked for roundness, and were found to be perfectly true.
I had thought that I had a bad injector, but they're all fine. The harmonic balancer is new and installed correctly, and there are no other engine issues I can find. We've determined that it definitely is not TC shudder or flex plate related.
Current suspects are motor mounts, trans mount, or incorrect u-joint angles -- and even possibly that the driveshaft was balanced incorrectly.
Any other ideas where I should look?