My 2003 Buick LeSabre has been acting like the transmission is slipping when you first step on the gas, especially when making a turn at startup such as from a stop sign. Today for just a few seconds a message flashed that referred to the traction control. I haven't been able to replicate to get the message again. I don't think it'* the transmission. Car has 129,000 miles. There is also a slight wobble or vibration when driving. Any suggestions on where to start looking?

Check the air pressure in your tires. Both front should be equal and not over inflated. Use the drivers door specification Eg: 33psi.
The traction control is probably sensing a tire is slipping, take it easy in the turn & you should be fine.
As a side note, inspect the tire condition for even wear, good tread depth, as well as having front wheels aligned each year.
Check for play in bearing/hubs & Have the wheels checked for balance.

What Soft Ride said ^^^^^^^

. . . and: Have you checked the transmission fluid level?