It would help a ton if we knew what year and model your Bonneville was. Please edit your profile and put that in your signature.
In simple terms, even if your engine is warm, your trans is not. On cold mornings, your trans fluid viscosity is high, so when you shift, you get that effect. Make sure your rpm'* are low if this continues to be a problem (to prevent damage to the stovepipe or reverse reaction drum).
With my shift kit installed and my adjustable modulator maxed out on one of my cars, I have this problem, but not as bad as before the rebuild. I've learned to roll out of the driveway, then shift halfway down, or simply take my foot off the brake quickly to prevent the reverse shift from impacting a non-moving set of axles and diff.