What service should be performed at 90,000 miles on a 2003 GMC Envoy XL?

Repeat service from 30,000 miles. replace all filters and hoses, engine and transmission. inspect/check differential oil also.

Have general check up of your car, engine check, oil level, ignition, smoke emission, lights (all), suspension
systems, ball joints, steering systems, propeller shat bearings, gear oil level, differential gear oil level, brakes
systems (pad, drum, disc, brake fluid level), transmission systems. etc......

I would look into a tune-up as the spark plugs are a common cause of issues on this vehicle. I would at the very least remove the plugs and inspect them. If they are fine then it may be a clogged fuel filter.
Check the coolant and oil level