Donald Morrison believes the chilling anonymous letter was linked to his campaign to discover the truth about the lawyer's mysterious demise in 1985.
The stencilled message reading: "Let's hope the next victim of a police shooting is you," was posted from Glasgow to his home in Banffshire two years ago.
After filing a complaint with the police, the 74-year-old then began to suspect that he was being followed and also believes his phone and email have been hacked.
Donald Morrison, inset, friend of Willie McRae has received a death threat
Mr Morrison said he had decided to speak out about the threat against his life as the 32nd anniversary of his friend's death approaches next month.
I have reported two incidents to the police but there has been no interest to follow them up
McRae was ruled to have shot himself on his way to his cottage in Wester Ross, although the revolver had been fired twice and was found in a burn some 15ft from his car.
Mr Morrison has always maintained that he spoke with the 62-year-old in Glasgow city centre on April 5, 1985, shortly before he left for the Highlands, and noticed he was being followed by two cars.
The former Strathclyde PC said: "I have reported two incidents to the police but there has been no interest to follow them up.
Donald Morrison says he saw McRae being followed by two cars on April 5, 1985
"The first one was a motorcyclist who followed me on a road to Fraserburgh for 59 miles back in 2015 and the second time was in Glasgow last summer. On both occasions my vehicle was interfered with while I was away from it although nothing was taken and I could
not see any obvious damage or trackers or such like.
"I have also received a threatening letter which stated, 'Let's hope the next victim of a police shooting is you'. The first three words were handwritten, the rest stencilled in. It was quite unnerving.
"I also believe my phone and computer have been hacked and it is all down to the fact I will not give up until I have answers for Willie McRae. I feel I owe him the truth since no one else seems to care."
The death threat was postmarked in the Glasgow Mail Centre on March 9, 2015 – the day after a story appeared in the Scottish Sunday Express about the ongoing investigation into McRae's death.
Mr Morrison has been in touch with Glasgow-based solicitors Beltrami & Co to discuss his concerns while another solicitor in Aberdeen has a copy of his signed affidavit should anything happen to him.
McRae was ruled to have shot himself on his way to his cottage in Wester Ross
The pensioner has always insisted that McRae was in good spirits before his fateful journey north.
He had just bought two bottles of whisky for his weekend away and told Mr Morrison, "I've got them this time," while patting his briefcase full of documents.
Although the ex-policeman initially thought the papers were about dumping of nuclear waste, he has since come to believe the lawyer had unearthed evidence of a powerful establishment paedophile ring.
Mr Morrison said: "Knowing what we do know of the sex abuse and the names of some of the perpetrators it is probable Willie McRae had come across some explosive evidence which is why he had to be silenced.
"I do not accept the suicide verdict. I know what I saw the night I stopped the traffic to allow him to perform a U-turn on his way up north. That was not a man about to take his own life.
"I saw two cars with two men tearing after him, Special Branch, and 24 hours later he was dead."
A public petition to launch an official inquiry into McRae's death was dropped after officials insisted there was nothing further to investigate.