I have a vibration at 50 mph in my '97 Defender. It's felt strongest through the steering wheel, but it's evident in the floor, etc. to a lesser degree. It comes in waves, slowly building up then fading out completely in about a 5 second cycle, followed by 5 seconds of no vibration, then repeating.

I recently replaced the differential, all the U joints, balanced and rotated the tires, and had the front end aligned, but still it persists.

Any ideas?