I have a hydrant that leaks from underground when left on. Is this something that can be fixed from above ground or will I most likely need to dig it up?

They are made to be fixed from above ground. You can get a valve repair kit. Its a big rubber bullet and some O-rings. The bullet goes on the end of a metal rod that goes from the top operating handle down into the pipe. There are handy youtube videos on how to do the repair. You just have to find the kit for your specific unit. I think they ran me like $10.

I rebuilt ours from above, still leaked.  Bought a new one to replace it with.  Dug down and found threads leaking at an elbow, not the hydrant itself.  I've heard the drain can get blocked which can allow things to freeze and break.  When I put everything back together I took 2 5-gallon buckets and cut half of the bottom from each.  Inverted one over the hydrant and filled it with gravel.  Inverted the 2nd bucket and clocked it 180º from the 1st.  Zip tied some landscape fabric to the hydrant pipe to cover the buckets.  I've had ZERO issues with it 10 yrs later.

As a general rule if your hydrant leaks at the handle or the threading from the vertical pipe to the handle repairing it is simple, remove the hydrant handle with a couple of pipe wrenches replace the seals, new pipe tape and reinstall.

Water percolating from below ground is going to require excavating enough soil to get to the 90 degree elbow from your horizontal supply line.  It's either going to be a hole in one of the pipes or the elbow has failed.

We've got six of them around the ranch, every time one starts to leak I pray it's at the handle.

If you've got to dig down to fix the pipe leak, get the rebuild kit for the hydrant, since it's going to be off anyway it's a good time to rebuild it.

Well I tried to do the repair from the top but it was still leaking. 

Looks like Ike I found the culprit!

That'll do it.

Wrap it in duct tape. It'll be ok

That is an unusual failure. Looks like it froze. Is it deep enough for your area?

It is deep enough but it does look like it froze 

some of it might be man made damage though 

I've got two I need to dig up and fix... ugh.