In a diesel the engine mounts get quite a workout. Don't skip the engine shock. Remove it and make sure it still has resitance. If it allows the shaft to move quickly then replace the shock. Also your engine/tranny mount has a transmission mount at the rear of the tranny. Check it also.
20 years is a little to long between engine mount changes. Check them every 3 years, they may not need replacing but this way your on top of them. If you have a lot of fluid leaking, oil, deisel, pwr steering etc. The petrolium products will cause the rubber to brake down. Then anything around the engine that is rubber will have its life shortened. So fix those leaks and clean the engine on a regular basis.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car