The 25-year-old ex-Newcastle striker was handed an eight-month jail term two weeks ago for conspiracy to defraud.
It is illegal for prisoners in British jails to post from social media accounts while behind bars or to have mobile phones, although it is unclear whether it was Ranger who uploaded the post.
Tiote tragically died aged 30 after collapsing in training with his team Beijing Enterprises earlier this week.
The Instagram post read: “Sad to hear the news about my brother Cheick Tiote.
“He would go through a brick wall and give you a two footed tackle in training a day before a cup final!
“What a guy though, top man and a professional. My condolences go out to his family.”
The post attracted more than 2,400 likes with some users questioning whether Ranger had written it from behind bars.
A spokesman for the Ministry of Defence told the Southend Echo: “This behaviour is unacceptable and we are working to get the content removed from social media. The prison is also investigating.
“It is a criminal offence to bring a mobile phone into prison, or transmit sounds or images from within a prison using a mobile phone. These offences carry a maximum penalty of two years in prison.”
Ranger started his career on Tyneside, scoring three goals in 62 games for the Magpies.
Loan spells with Barnsley and Sheffield Wednesday followed before he enjoyed the most prolific spell of his career at Swindon Town, netting 10 times in 28 games for the Robins.
He left the County Ground for Blackpool before moving to Southend where he played 25 times last season, scoring eight goals.