I bought an old used pickup truck a few months ago that has a trailer hitch
with a lock on it. I have the keys to the lock, but I can't get the lock to
open so I can remove the towbar/ball (or whatever that is called).

Here's a link to an online image of a lock that looks like the one I have:

http://www.pacifictrailers.com/index...4,s :0,i:110/

Hopefully, the link will work.

The lock has a press-type button on the end. The key goes in, and
everything seems to turn freely, etc. But I can't get the lock to come off
so I can pull the bar out and remove the towbar/ball.

I have used oil etc. to get everything loose, and I have tried tapping on
the mechanism. It just won't come apart. If I can't figure it out, I guess
I'll have to cut the lock off.

Does anyone have any suggestions of what else I could try?