I am very well-versed in this phenomenon. I also thought it was dead in 1989.
The influx of rock and rap has been going on one way or another for ages. One could argue that the likes of George Clinton and Jimmy Castor had a lot to do with it as they mixed several ganres into a blender and made it digestable for people on both sides of the fence.
As far as actualy rap/rock crossovers, some highlights in almost chronological order:
The first rap song (arguable) "Rappers Delight, featured the bassist who would go on to play in Tackhead and Living Colour.
The first Public Enemy album Yo! Bum Rush The Show featured the guitar talents of Vernon Reid, of Living Colour, on a few cuts.
(Please note that contrary to popular opinion, Living Colour was NOT a rap/rock act. They were a ROCK act, period. They just happened to be black.)
San Francisco thrash band Mordred has a DJ in the band, and they covered “Superfreak.”
“Superfreak” was also covered by a Rick Rubin band called Hose, which he put out on Def Jam when he was still a student in his NYU dorm. It pops up in collectors stores from time to time. It also sucks…
Rubin had Public Enemy do a song “She Watch Channel Zero” over a Slayer song. Kerry King from Slayer did a guitar solo on "No Sleep 'Til Brooklyn (named after a Motorhead album) and appeared in videos for The Beastie Boys.
Early Run-DMC albums sampled all kinds of rock stuff. Billy Squire’s “Big Beat” was a staple in any CDs crate during this time.
Aerosimith and Run-DMC.
Anthrax and Public Enemy.
KRS-One rapping on Sonic Youth’s Goo album.
There are tons more…