Back end seems low... Raise it? Or lower the front?

I would only worry about it if you were hauling a trailer or heavy loads. From the factory truck beds are higher than the front to compensate for the weight in the back. Kinda like a flatbed semi-trailer and how it is arched instead of flat. Most likely your leaf springs are starting to show their age. I wouldn't raise the back or the front unless you absolutely want a level truck. If you wanted to replace anything I would recommend replacing the leaf springs and shocks. It should raise the back to factory height if you get the correct parts. Another thing to note, the factory leaf springs on my 94 S10 are strong as ****. I put enough scrap metal in the bed to basically fully flatten the leafs. My tailpipe would scrape when I reversed out of my driveway. Once I dropped the metal off, they returned back to normal with no problems. I think the total weight of scrap metal was around 1000 pounds. Can't remember the exact weight. Anyways, that is my recommendation.
You have a great day,
Gavin