Post some pics and maybe folks can guess what killed them. Any strange weather? How do the others look?

I say wait until the fall to decide what to do. It is better to plant then than June.

if other trees in the row die by the fall then you know some bug or infection is present and Rose of Sharon is not a good choice for a replacement.

"...they are 5-6 years old now, and I don't know if they replant well."

I've replanted very large, mature Hibiscus syriacus (assuming that's the plant you are talking about) without any problem. They transplant easily. As mentioned about, though, this is not the time to do it.

Planting a Tree or Shrub

I have a brown thumb, and I've successfully transplanted Rose of Sharon. They transplant very easily...and the produce so many volunteers, if you just refrain from using the weedwacker and lawn mower for a while, you should have plenty of potential replacements to experiment with.

Personally, I've had more luck transplanting them in Spring then in Fall.

Did you have an especially bad winter? Did you get a lot of wind? Were these two in a less sheltered spot? I think cold and wind can knock them out.

Thank you everyone for your comments. I do have MANY potential replacements that are smaller, but also have a few 5-6 years old ones I can use. I will give it a try in Fall or Spring.

Interestingly enough, I think I figured out what happened. I was going to inspect them, see if there was any green under the bark and they literally popped right off the ground, I don't know how they were staying upright, they were broken right above the roots and had been rotting. I had a bit of a flashback that last winter, 2013-2014 winter, I had piled a LOT of snow right in front of them, and my pile got quite high and I had to throw it high. I think I hit those trees directly a few times with heavy, wet snow. They were bending back from the weight in front of them. I probably damaged them then, and they died last year and rotted.