I'd like a checklist for a 90,000 mile tune-up.

There are 2 engine options for your GMC and 4WD and 4WD. The basic requirements from GM are change engine oil and filter, fuel and air filter and basic inspections. I would every 30k change all fluids, coolant, engine oil and filter, trans fluid, brake fluid ect.... At 90k I would also probably chance the spark plugs.