How can i give a joint Limited Strength?

I'm trying to make rigidbody click and drag. That is, clicking and dragging any rigidbody in the scene will spawn a little sphere with a joint, and latch that joint onto the clicked rigidbody. Then the sphere will follow the cursor, and drag the latched object along with it.

I can do all of this very easily with a fixedJoint, and i could lift and drag anything without problems. But, i don't want the player to be able to lift ANYTHING

What i'd like, is to be able to give this pulling some kind of limited strength (i don't want the joint to break) so that although small objects will be lifted and carried around, heavier ones will only be dragged along the floor, and really heavy ones will just not move at all.

What is the best way to accomplish this? I'm looking at a Configureable joint, and the array of options is mind boggling, i need a little help narrowing it down.