default axis steps per unit Makerbot replicator 2
The Replicator 2 and 2x use 18 tooth GT2 pulleys, 1/16 microstepping, and 200 steps/rev steppers. That makes the proper steps/mm value 88.888889.
Note that Makerbot used ~88.56 steps/mm in their defaults, which is the value you get if you calculate from the belt+pulley pitch diameter from the Gates GT2 specs. But this is the value you use for calculating belt length required in a closed loop, not for steps/mm. The 88.88... number is more accurate.
Z is 400 steps/mm. (8mm lead, 1/16 microstepping, 200 steps/mm motor.) Sidenote, make sure you don't over-drive the Z stepper with plug-in drivers, it's only rated to about 0.4A.