I have my 91 LS400 starter apart now. I jacked the rear coolant bypass pipe up enough to get tools on the two bolts. It was very slow and difficult removing the two starter mounting bolts. It is not necessary to remove the rear bypass completely, just jack it up without removing it. The wiring harness makes removing the rear bypass pipe almost impossible.
The problem with the starter appears to have been the ground side starter contact which was worn down so thin it was making intermittent contact. The positive side contact and the plunger contact plate appear to be worn only slightly.
The brushes and commutator are still in good condition, but it was definitely time to remove a huge buildup of brush dust which would have caused problems soon.
After 13 years and 240,000Km the starter does need attention. It is too bad it is such a huge job to do.
Putting it all back together is going to be a challenge, but I photographed things as they came apart so with patience it can be done.