Shakes while accelerating uphill. How to fix?

I've got a '96 Disco with 96K miles on it. It just started doing this. When going uphill it starts shaking....it shudders. We've replaced the front u-joints, they showed no signs of wear. Greased the CV joints, tuned it up with quality plug wires & spark plugs. The harmonic balance is maybe off? The motor mounts look fine, the harmonic balancer has some small,tiny cracks in the rubber. The rear coupler was replaced less than a year ago and is cracked again, but not bad. So all of this adds up to .......what? Why the shuddering? Any ideas on this?