Best way to stabilise a heavy gate that's sagging?

[RigidRaider]

My neighbour has two heavy hardwood gates, T&G plank panelled, which are sagging under their own weight and no longer square. Each gate is about 5' wide so the leverage is considerable. The timbers are not very tight together and they seem to have been assembled mostly with wood dowels. I think the only way to get them square again is to lift up the outside edges to slightly above square then screw a diagonal timber from the top of the hinges to the bottom outer corner, which will brace them and might even pull them back together.

Any other suggestions? I don't want to disassemble the gates completely.

 

[Burnerman]

More or less on the right lines, I think.....are there any visible tenons that could be wedged? It could be an idea to brace the corners with galvanised steel bolted through.
How about fitting a jockey wheel on each gate, just to help with the weight?
John

 

[RigidRaider]

That's not a bad idea, thanks!