Speedo and shifting problem

[ChrisCTtahoe]

I have a '08 tahoe 4wd with 5.3. It just started having a strange problem. Speedo reads fine except when you come to a stop. At a full stop it reads 20mph. When this happens it will not let the transmission fully downshift to 1st. When you go to take off it lags because it is starting in 2nd gear until you get over 20mph so it can start acting normal. I have just been manually shifting into 1st at stops to work around the problem. Speed sensor problem???