Adding a spring lift is all upward travel. The only time you'll run into problems with the stock driveshaft and the exhaust crossover is if you add downward travel, like you'd get from installing a set of Bilstein 5100's.
However (you knew that was coming, didn't you), if you add TOO MUCH lift, even if it's just springs, you'll be putting your stock ds at too great an angle, and eventually it'll fail. It might last six months, a year, two years...Depends largely on how much stress you put on it wheeling, how you drive, etc. The TeraFlex exhaust extension will not make a difference, in this case. It only attempts to fix the first scenario.
You should be fine with the 2.5" spring lift.