I've got an hold Wheel Horse lawn tractor (probably from the 80s) with
a 12 hp engine. I got it about 5 years ago, and except for the
problem of mice building nests in the engine compartment, it's been
pretty reliable.

Sometimes if the grass is a bit wet and I end up on too much of a
slope, the left rear wheel might slip and just spin.

That's no big deal, but I seem to recall when this would occasionally
happen in the past, it was both rear wheels that would spin. Now it's
just the rear left, and the rear right doesn't move - which means that
only the rear left is being powered. Maybe I'm misremembering, but I
swear this had 2-wheel drive in the past.

Yet, when I look down there at the axel, I don't see any obvious
missing bolt or locking pin or anything - like on my snow thrower.

Is it possible that whatever is broken is inside the axel somewhere?
I could use some advice on what I should be looking for to fix it.
I'm not afraid of taking this thing apart - but it would help to know
what I'm looking for. Or is it possible that it really is just one-
wheel drive?

Thanks for any advice.

-J