EAGLE Matched Length Pairs / Groups
Here's how I do equal length differential pair routing in Eagle CAD:


Name traces D_P and D_N (or something _N and _P - seems like Eagle CAD needs the suffix). 
Create net class in schematic and add both traces to it 
Route the traces, either together (the default) or separately (type ESC and Eagle CAD will stop routing the second trace).
In board view, type run length D_* in the command window
In the dialog box that comes up, click on longer pair length, copy it (Command-C works on my Mac)
Type meander [copied length]mm, but replace [copied length] with the value you just copied (Command-V to paste works on my Mac) into the command window. mm means millimeters, length.ulp measures in mm
Click on the shorter trace to meander it.
Move mouse to meander the shorter trace until its length is the same as the longer trace and the meander pattern is to your liking. 


The Eagle Cad v6 Manual p175-178 helps.