It'* becoming more of a possiblity in my mind. I know the LeSabre'* have a 2.84 axle ratio (if I recall), which means at the very least, your shift points are probably at the wrong spots. What other problems that may cause, I don't know.
Again, we've had problems posted with similar transplants twice before. That information is here, but I don't recall it having anything to do with TCC lockup. The problem you have is an unproven combination. You don't know the condition of the trans you installed, and you don't know how your PCM is dealing with it.
If the LeSabre PCM controls the TCC with a different wire, you're hosed unless you can figure it out. What you need is the factory service manuals for your car and the donor car for comparison.