I am bringing my 84 300 D in for service for the first time in 100,000 miles. What should I have done? I have maintained oil changes, belts, hoses, brakes, front end ,etc. but never had the engine serviced. What specifically does anyone recommend. Thanks

You really should have been doing valve clearance adjustments every 15,000 miles!!! So this should be # 1 on your list.

Have you changed transmission fluid and filter regularly...this is also very important to do every 30,000 miles. Drain fluid from BOTH drain plugs...one on transmission pan and one on torque converter just forward of the pan, remove pan, replace filter, and put pan back with new gasket. Easy to do yourself, but messy.

Also, have them check/adjust fuel delivery timing.

Prior to bringing into the shop, run a can of diesel purge through, then have shop pull the injectors and check them for release pressure and spray pattern. New injectors may be in order.

Sounds like you have kept up with the other stuff, not too much else for a shop to do.

Check timing chain stretch.

Sixto
91 300SE
81 300SD