Matthew Perry’s grieving Friends costars are determined to keep his legacy alive when celebrating the classic show’s 30th anniversary this year, but source say the current criminal probe into his death has cast a dark shroud over their plans, RadarOnline.com has learned. Sources snitched the surviving cast members — Jennifer Aniston, 55, Lisa Kudrow, 60, Courteney Cox, 60, David Schwimmer, 57, and Matt LeBlanc, 56 — are struggling to figure out how to mark the hit show’s anniversary without the sordid details of Matthew’s last days and death overshadowing the celebration. As we previously repo…