What do you suppose is causing the vibration of my '95 Caravan when cruising at 70 kph (40mph)?

Whenever you change ball joints(both ball joints should be replaced at the same time). You need to do an alignment and tires balancing otherwise vibration will occur. Bad tranny wil also give a different kind of vibration, not at the steering wheel.

most probable cause is the need to have your tires balanced.

have you're tires balanced and check for bent rim

maby unbalanced tires or wheel hub