Winter here was brutal with multiple days at -30F (at least two nights). This year I had no flowers. I have very few leaflets, and now some of those are withering.

Contender which is younger is better than Reliance which was overgrown and pruned back at the start of winter.

So, now -- I have a handful of leaves, do I wait some more, or chop them down and start over? Or is there something that can be done to stimulate the trees' growth?

I have two Reliance peaches that bore very well in 2013.
This year mine are like yours. Leafed late and very sparse.
Lots of dead wood. They are looking better now, but since they were 8 and 9 years old respectively, I am going to cut
them down and re-plant next year. Their age is against them even if they do recover in a year or two.

I would think there will be a run on peach trees for spring 2015 with all the damaged peach trees I hear about from people in various states. This is just homeowners not even
the losses in commercial orchards.

Where I work, we get in container peach trees from a grower. They had trouble overwintering the container fruits
too and had heavy losses.

I will try to place my 2015 spring order in a month or two
assuming the bare-root grower I like to deal with will accept
my order that early. I am afraid ordering late might be problem if the growers had damage in their bare-root fields
too on peach trees. Supply might be short.

If they are grafted trees and you know where the rootstock is, you could cut back to above that line and see if you get new shoots. but you will still be set back at least another season waiting for new branching/etc.

Spartan-

That has been my thought too. If you want trees next year, i'd order early! I think there is going to be a run on them. Michigan had huge losses and homeowners have had huge losses. I doubt there will be enough supply next season, but who knows.

Chris-

If that is a recent picture, that tree is toast. I doubt even pruning it back to the trunk would help. It might throw new shoots right from the ground, but who knows if that tree is grafted or not.

That tree looks really bad. This winter was just too much for a lot of these stonefruit.

But in my case because I grow roses, I always plant the graft on a rose bush below the soil line so the rose will overwinter, and grow out of the graft if nothing survives above ground, and probably did that here with the peaches trees, but it was a shallow planting nevertheless. So despite the fact I have a handful of leaves you suggest I cut them nearly to the ground to see what happens?
These trees were so vigorous last year, and produced an incredible crop. What a difference a real winter makes. Winter was absolutely brutal here.

The problem i see, if you have a situation like the above pics, where you have all this dead wood and then a bunch of leaves at the end...I'm pretty sure that isn't going to fill in over time..as in you will have vast areas of nothing, with growth at the very tops of the tree...which isn't going to work well going forward. My suggestion is just to try to start over down low and get yourself some new growth that is nice and lush....if that is possible. Or drop the tree and start over.

What is interesting, is i have some seedling trees here (3 of them) that are 3 years old now and look great..i had very little dieback. Almost like younger trees handled the cold better.

What you should have done is last year seeded a bunch of that fruit and now you'd have all kinds of replacements. From what i've seen, they grow pretty true (self pollinating) from seed. I'm guessing here, but i bet seedling trees are probably hardier then grafted trees....although i'm not sure if any real research has shown that.