That's a newer GM thing...97-03 cars will only shift out of 1st...every other gear you can hold.
As for shifting to 4th at the traps...something is up...because 93mph on the stock 2.93's is still in 2nd or barely in 3rd. Something is up transmission wise there...try leaving it in 2nd...and then put it in 3 when it gets close to redline.