This isn't true. IFR certified GPS units generally don't have Differential receivers. The main reason for this as far as I can tell is integrity. To be IFR certified, the receiver has to be able to tell when a satellite is giving bad data. In the older units, this was done with RAIM, which basically compares what is coming in from all of the satellites, and determines if all of the satellites are providing positioning that is relatively consistent. DGPS is a relatively simple system, and there's no way to check the DGPS signal for integrity. Without the ability to check for integrity, the additional accuracy is useless, as the design of instrument approaches is based on worst-case scenarios.

Now newer IFR certified units do have WAAS, which is similar to DGPS, except the signal is transmitted from a satellite, and is based on measurements from a couple dozen ground stations, the correction interpolated in the unit.

There has been a proposal to implement a system called LAAS for a number of years, which would be like DGPS, but would have the integrity checks that are required for aviation. However, it never really got off the ground.

As it stands, with WAAS, you can be guided to 200 feet above the runway, but this is not as good as the old analog radio ILS, which, if setup in a sophisticated way, can take a plane all the way to the surface.