If it shifts down to 5th when unloaded on the slightest of hills, I'd say you've got something wrong (unless you've got big tires). See my thread "Down on Power" on this page as mine started doing this too but only after about 7-8 months of ownership. Currently, my truck is at the dealer for the second time, last week they just reflashed the PCM but it didn't fix a thing. I'll update it when I get the truck back.
As for the rest of your question, I admit it is kind of confusing and I'm not sure I'm following you. You start off by saying you "drop it into 5th" which makes it sound like you know exactly what the plus or minus button is for. Basically, it allows you to choose the highest gear the trans will shift to and it will shift all other gears as normal up to that point. If I remember correctly, the + and - are only an indication of which way you can go (up, down or both), but since I don't have my truck right now, I can't verify.....but really, the number is all that matters. If it says 4 and 5, that means you are in 4th but it is set to a max of 5th.