Jim Moretti at Lynch Hummer was good enough to fax me exactly what is done for a 12K service:

1. Change engine oil and filter
2. Clean or replace air filter
3. Replace fuel flter (gas or diesel)
4. "Flush" cooling system
5. Change transmission fluid and filter
6. Change transfer case fluid
7. Change geared hub fluids
8. Change axle fluids
9. Check/Service batteries
10. Inspect belts/hoses
11. Inspect exhaust system
12. Inspect/rotate/balance tires
13. Check level/condition of fluids for brakes and power steering
14. Inspect drivetrain/CV boots for leaking seals or damage
15. Inspect control arms, springs, shocks, tie rods, idler arm, pitman arm, center link and ball joints for wear or damage
16. Check/lube all grease fittings & oil can points
17. Inspect all vent lines and fittings for damage
18. Inspect U-Joints for freeplay/damage
19. Inspect all transmission and t-case linkage fo bends or excessive play
20. Inspect engine mounts & insulators for poor fit or damage
21. Check wheel alignment
22. Check brakes FR.%____ RR.%____

Whew! no wonder it's so expensive!

Thanks go to Jim Moretti for the info.