I can't speak for other airlines, but as of today, AA's 75/76s are about half Pegasus and half Basic (IRU/DME). However, there is a time frame - I'm not sure of the exact date - where IRU navigation will become obsolete, and everyone must transition to GPS.
That date is January 1st, 2020 when ADS-B becomes mandatory from liftoff to touchdown at major airports and class A and B airspace in the US. The accuracy requirements are such that WAAS or dual frequency GPS's will be the only navigation systems capable besides all the datalink reporting requirements.