As long as all your fluids have been coming out reasonably clean, at 150K I'd do a detailed check for leaks, check major electrical connectors for any incipient corrosion (and clean them anyhow - especially any bulkhead connectors that tend to get overpacked with grease at the factory...) check front end/steering goodies, and check as many sensors as possible. It's a cinch you're due for HEGO sensors (if you haven't changed them already) and the rest could probably bear checking - all but the HEGO can usually be checked readily with a decent thermometer, pot of water, hair dryer, and a good ohmmeter (I tend to prefer an analogue meter for variable resistance checks, since it's more likely to show up any flat spots in the sensor that will mandate replacement.)
I have a 1994 FSM if you need test specs...
5-90
