I just put my 2nd rifle together last week. I bought a PSA mil spec buffer tube assembly and a mil spec Minimalist stock.

The problem is, I have to pull relatively hard to get the stock to move back and forth on the butter tube.

One friend suggested putting some lube on the buffer tube, which I did, and it helped for a couple of days and now it's back to being stubborn.  He also suggested doing some very fine sanding on the inside of the stock, only doing little amounts at a time.

Any other suggestions before I start sanding and possibly regretting it later?

Try another tube.  I had the same thing happen with a BCM tube, way too tight.  I replaced the tube with a VLTOR and the fit was perfect.  The BCM tube has been too tight on every stock I have tried it on, so you might try another tube before modifying anything.

You might just have to break it in some. I like a tight fit which is why I use LMT receiver extensions. Get some fine grit sandpaper and EVENLY sand down the buffer tube. Very slightly, a little at a time.

Or just buy a different receiver extension.

Aeroprecision makes a good one that is pretty cheap.

The MFT Minimalist is SUPPOSED to be tight. One of the reasons people love them is exactly that, super solid.

The Aero buffer tube is 6061 and the PSA buffer tube is 7075 and cheaper.  I have both and all my Magpul and B5 stocks fit the same on both.  The minimalist stock I have fits tighter on both but that's one of the reasons people like them.

I would not sand down the buffer tube.  Try a Brillo pad first to smooth it out and use the same Brillo pad inside the stock.  If that does not help, sand the inside of the stock.  

I thought maybe that was one of the reasons, but it's beyond what I'd think most people would like. I have to yank on it to get the stock to move back or forth one position.

I'd look for any imperfections inside the stock and adjust as needed, but would hesitate wanting to sand in the inside of stock as you would likely end up rougher than when it was manufactured.  Wet sanding the tube would likely smooth it out if it is rough using 1000 or 1500 grit.

I'd rather have a stock that's too tight than one that's all wobbly. That said, I haven't handled yours so, it's hard to judge just how tight it is. Personally, if it's not hurting anything, I'd just run it like it is. It will likely loosen up a bit over time and how often do you really need to adjust it anyway? You'll probably have a tough time evenly sanding the inside of the stock. You might try working it back and forth for a while while you are sitting around watching tv and see if the parts wearing on each other loosens it up a bit. As a bonus, this should annoy the shit out of your wife.

edit: After working the parts back and forth for a while, if it's still binding, you should be able to see some wear marks inside the stock if there is an imperfection that's causing the pressure on the tube. Then you can lightly sand that spot until it moves a bit more freely.