Which D position best to use.

[muncher]

Hi All. Hope this doesn't sound like a daft question. I have a 87 GMC Sierra Stepside. 305 engine. The trans selector has two D positions, one with a circle around (overdrive). My drive to work is mostly following traffic and not a lot of free moving highway. I find that my trans wants to drop in to last gear way too soon. Then when I accel it changes up then when I even off it changes back in to top again. Because of the traffic going varying speeds this constant changing in to the last two gears gets annoying. So my solution is to put it in to D (with the circle around it) This way the last gear is avoided. Is this the way to go or is there an adjustment I can do to stop the last gear selecting too soon.