If I was contacted by a private investigator about a bad check and was told I have 24 to 48 hours to pay or a warrant will be issued, is that legal?

Well, that depends.  I would say that if you were contacted by a private investigator rather than a law enforcement officer that this is a scare tactic.  But I would take the matter seriously because if and when it is reported I am sure that a warrant could be issued.  There is a new law as I understand it going in to effect in September 30, 2011 regarding "passing bad checks.'  Here are the penalties:

"Whoever violates this section is guilty of passing bad checks. Except as otherwise provided in this division, passing bad checks is a misdemeanor of the first degree. If the check or checks or other negotiable instrument or instruments are issued or transferred to a single vendor or single other person for the payment of five hundred dollars or more but less than five thousand dollars or if the check or checks or other negotiable instrument or instruments are issued or transferred to multiple vendors or persons for the payment of one thousand dollars or more but less than five thousand dollars, passing bad checks is a felony of the fifth degree. If the check or checks or other negotiable instrument or instruments are for the payment of five thousand dollars or more but less than one hundred thousand dollars, passing bad checks is a felony of the fourth degree. If the check or checks or other negotiable instrument or instruments are for the payment of one hundred thousand dollars or more, passing bad checks is a felony of the third degree."

Your check was written prior to now but I would still take the matter seriously and contact an attorney.  Good luck.