Do I bond ground and neutral for a feed from a disconnect?
Since it's a subpanel, neutral and ground should be put on separate bars.  That's always the case. 

If you are certain you have an ungrounded 3-wire feed, then you can jumper neutral and ground at the panel.  However you should be prepared to easily remove that jumper.  

Not all grounds are wires. Metal conduit is a grounding path.

As soon as feasible, run a ground wire.  It doesn't need to be in the same cable or route as the feed.  If it's conduit, it can be fished with some difficulty. It also doesn't need to enter the house, if the house has a ground rod, it can go there instead.   As soon as you have retrofitted, remove the neutral-ground jumper. 

Remember the detached outbuilding also needs its own ground rod.