Should I ask my dr. for it and request an appt. with him to discuss the situation?
Absolutely. Tell him not to reply until you speak with him/her. If your doctor replies and agrees with the second opinion, you have blown the chances of getting a referee examination because there will be no conflict in the medical evidence requiring the scheduling of a neutral referee. That is what you stand to lose. Of course, if your doctor actually agrees with the second opinion, he/she ought make that fact known in a letter to OWCP.
If you are physically capable of returning to work in either a full or modified capacity, this is what you need to do. If this is not the case, and you cannot work, then you must prove this by the the weight of the medical evidence.
Forget about vocational rehabilitation until it is determined that you are partially disabled. If the USPS does not want you back, vocational rehab., what whatever it is worth, will start. You can deal with that aspect of your case separately. Concentrate on one thing at a time.