AirPort Express 5 GHz only?

If you don't see the 5 GHz network as an option to "extend", the signal is probably too weak at the location of the AirPort Express. 5 GHz signals are absorbed much more quickly by walls or any obstructions. Unless you literally have good "line-of-sight" between the AirPort Extreme and the AirPort Express, extending 5 GHz is probably not going to work.

Open AirPort Utility - Click Manual Setup
Click the Wireless tab below the row of icons
Wireless Mode = Extend a wireless network
Wireless Network Name = The name of your 5 GHz should appear here. Try manually entering it if it does not appear as a choice to extend.

Unfortunately, you will probably not be able to get what you want.

If you have one name assigned to both the 2.4 GHz and 5 GHz bands on your AirPort Extreme, the AirPort Express will choose the best quality signal that it "sees". That's usually going to be the 2.4 GHz signal because the strength of a 5 GHz signal falls off much more quickly compared to 2.4 GHz

My AirPort Express will extend the 5 GHz band from my AirPort Extreme when the two networks have the same name as long as I'm within about 12-15 feet of the AirPort Extreme. As soon as I move the Express further away and around the corner, it starts to extend the 2.4 GHz band.

The only way to "force" the Express to extend the 5 GHz band is assign a separate name to it and then have the Express extend that band. Same with your computers...the only way to force a computer to connect to the 5 GHz band is perform the same type of setup.

I have kind of the same issue where I get full speed off my TC but very sluggish from the AExpress. I am trying to extend the network of my TC. Would getting an AExtreme that connects to the TC help?

+Would getting an AExtreme that connects to the TC help?+

Probably not since the Express and the Extreme are both rated for the same radio power. Try positioning the Express closer to the main router. Most users place it way too far away.

Assuming that there are no major obstructions between the main router and the AirPort Express, a good starting point for the location of the Express is a point approximately half way between the main router and the area that needs more wireless coverage.

An average wall in a home will, on average, absorb 15-25% of the signal and a ceiling will usually work out at 20-35% since it is much thicker. If the signal from the main router has to travel through 3-4 walls or a ceiling and a wall to get to the Express, there's not going to much of a signal left for the Express to extend. And it will only extend what it receives.