To determine a pulse period from RXTE data or any other X-ray satellite, I recommend using tools provided within the AIT astronomy library. All routines are well documented in the header of the routines, please check there for the relevant parameters. A quick recipe, based on epoch folding lightcurves, is the following:

1. Read the barycentric lightcurve, preferably with IDL's readlc procedure. Use the /MJD keyword to obtain the time stamps in units of the modified Julian date.
2. If known, correct the lightcurves for the motion of the X-ray pulsar around the center of mass of the binary system, using IDL's binarycor procedure.
3. Use epfold to epoch fold the lightcurve.
4. Using the best period, use pfold to obtain a pulse profile.

For event data, it is often better to work directly off the raw data and not generate lightcurves. For this, use the following recipe:

1. Read all event files using mrdfits from the GSFC astronomy library.
2. Convert the time tags into MJD with met2jd (do not forget the /MJD switch).
3. Barycenter the times with barycen. For this you will need the spacecraft's orbit information. For RXTE the name of the spacecraft orbit file can be found in filter/orbit.txt if you have previously run pcaextract. These orbit files can be read with readorbit. Be careful with the units (sometimes orbitfiles are in km, sometimes not). This step will take some time.
4. If the binary orbit is known, correct the barycentered times for the motion of the X-ray pulsar using binarycor as above.
5. Use epfold to epoch fold the data.
6. Use pfold to obtain a pulse profile.

I will soon augment this page with example IDL programs.