1981 Volkswagen Rabbit Diesel motor shaking the whole picku

I bought a 1986 jetta 1.6 diesel engine and had it put in the pickup. The engine ran smooth in the Jetta but when put in my pickup it shakes the whole car. My mechanic indicates the Jetta had a different motor mount and he wants to improvise a motor mount in the pick up to match the Jetta mounts. He thinks that will stop the rough idle and shaking of the car. When on the road running above 10 mph the motor runs pretty smooth. The original diesel engine did the same thing. We did put new motor mounts in the pickup and it did not improve the shaking. The engine is pretty much the same engine that was in the pickup before the change.