2 thoughts:

1. This is the beginning of your transmission going out. This is how mine began showing signs before it started grinding going between third and OD. This problem would require a rebuild.

2. You have a slight missfire that makes it feel as if the tranny is slipping. Misfires commonly occur when under load at low RPM's.

I think #1 is more likely, so I would drive with OD off until you can get it looked at or you can just drive with it off if you can stomach the increase in the gas bill. You are likely looking at $2000 if it is the tranny. Sorry, but 130,000 is probably around average for a truck tranny that tows often.