Definitely not a TC issue. The TC is always going, as long as the engine is turning. The only thing that ever changes in the TC is the lock up at a certain speed. Even if it failed to lock up, you would still get forward motion, just at a slightly higher rpm than if it had locked.
Does it just hit 45, then the rpms go up without the road speed going up?
Have you tried manually putting it in 2nd, and see how it behaves?
Try driving in manual 2nd, and then keeping it in manual 3rd (if that'* an option), just don't put it in OD.
If you're hitting this "wall" at 45, and the rpms jump up, the trans isn't going into the next gear. Depending on how hard you are accelerating, that could be 3rd or maybe OD.
Did this suddenly happen, or has it been getting worse over time?
2004 Buick LeSabre Custom FE5 GXP suspension conversion. 170Kmi
2001 Silverado 4WD 5.3L, 33" BFG ATs, Z71/Z82 pkg, 147kmi.