In 1991/1992 his vocal delivery was like that because he didn't give a f*...
He was bored to death by October/November 1991: shortened the setlist and became incredibly static for the reminder of the tour, gave up playing the beautiful outro in R&J with the Pensa, etc.
So, I'm not sure his voice deteriorated that much after 1985, he just didn't want to be there so he put absolutely zero effort in his vocal performances.
Telegraph Road in 1992 went just like this:
believe in me. I'll get you away. out of the darkness into the day.
these rivers of headlights. rivers of rain.
anger that lives on the streets with these names.
run every red light. memory lane. desperation explode into flames. don't want to see it again.
I might have put more effort typing this than Mark singing it back then