I own a 1986 Cadillac Sedan deVille. (FWD, 4.1 L engine)

How can I modify the car so that it will be a RWD vehicle?

The easies and most logical way of converting it to RWD is by selling it and getting a Lincoln LS/TownCar.

Got that right. There is no way to convert a front wheel drive to a rear wheel drive without some MAJOR modifications. Like spending some $30,000+ on the car. And for that, you can get something else. Probably the cheapest way would be to lift the body off your car and put it on a rear wheel drive frame if you could find one that fit.