Can I be fired for being taken out of work with a doctor's order from an accident injury?

You weren't hurt on the job and the employer needs the work performed. For what it is worth, your lost wages are another damage the mail truck should pay for.

The answer to your question depends upon whether the Family & Medical Leave Act ("FMLA") applies to the situation. For the FMLA to apply, the following conditions must be satisfied: [1] You had to work for the company for at least 12 months prior to the beginning of the leave; [2] the employer must have 50 or more employees at your worksite, or within a 75 mile radius of your worksite; [3] you must have given the employer adequate and timely notice of the need for leave under the FMLA (although one is not usually required to mention the FMLA by name, nor is one usually required to use the words "leave" or "leaves of absence"); [4] you must be able to return to work within 12 weeks of the beginning of the leave; [5] you had to have worked at least 1250 hours in the 12 months prior to the beginning of the leave (which usually averages about 25 hours per week); [6] you must have timely submitted to the employer any completed forms that the employer asked you to have completed and submitted back to it; and [7] your medical condition must meet one of the definitions of a "serious health condition" under the Department of Labor regulations applicable to the FMLA (which your condition sounds like it would). If all of these conditions are met, and the employer did not eliminate the job position for reasons unrelated to the taking leave, then there could have a claim under the FMLA. You should consult with an attorney who concentrates his/her practice on the handling of employment discrimination cases, and who has substantial experience with FMLA cases in particular. It has been my observation that not all employment discrimination lawyers have a good grasp of the FMLA and/or the Department of Labor regulations which apply to the FMLA.

Your doctor has no authority here. You have no rights here. In GA, an employer is not required to hold your position for you, with or without a doctors note. You may be eligible for Family Medical Leave Act / FMLA, which does not hold job, but does permit you unpaid leave, and right to return, if the employer has an opening. Otherwise, completely legal to discharge you for this.

Immediately contact an attorney.

That is their right. If you are not doing your job someone has to. If the did not need the job done there would not be one. Sorry, your insurance is you remedy in Michigan.