In the late 1980s, what caused the rehnquist court to start to restrict congress's ability to regulate various areas of the economy?

changes in the composition of the Supreme Court.  

This was largely due to efforts of President Reagan whose appointments of Justices to the Supreme Court made the Court largely conservative. This includes elevating  William Rehnquist, to the position of Chief Justice in 1986 when Warren Burger retired

what was the significance of the plessy vs ferguson ruling and jim crow laws

answer the impact. plessy v. ferguson allowed 'separate but equal, also known as segregation, to become law in the united states