I would like to know if I can converte my rear drum to rear disc brakes?

Yes, it can be done but it's not practical. Drum brakes require a lot less effort to make them work because due to their design, they tend to self-energize with very little pedal effort. Disc brakes require much higher pedal effort. That's the main reason power brakes became necessary. Drum brakes will already lock up the wheels and cause a skid. How much more stopping power do you want?

The front and rear brakes are very carefully balanced in the design of the vehicle. It's also very likely you have a height-sensing proportioning valve that varies the rear braking pressure based on the load in the back of the vehicle. They can't design the brake system to handle the wide range of no load to full load in minivans and trucks without using that variable valve.

You would need to replace the master cylinder. The port for the rear brakes has a residual check valve to maintain about 10 psi on the rear hydraulic system. If that master cylinder were used with disc brakes, the calipers would always be applied. The brakes would overheat leading to fade, they'd wear out quickly, and fuel mileage would plummet.

You would need a different combination valve. They are tailored to the specific vehicle and will be different for many variables including the extra weight of an air conditioning system, different wheel base, and other options. You'll be replacing the entire rear axle assembly to get the proper mounting system for the calipers. The parking brake cables will be different.

The main reason for going to rear disc brakes is calipers release faster than shoes. That makes them easier to design an anti-lock system around. Some older ABS systems pulse the brake for the locking wheel up to 30 times per second. Shoes are too heavy to retract that quickly.

Disc brakes DO cool better under hard braking conditions. That leads to reduced brake fade when coming down out of the mountains.

I have rear disc brakes on my '93 Dynasty but it has anti-lock brakes too. With that system you can just about tear the seat belts off the hinges in a hard stop, but when driving normally, it's impossible to tell any difference between my car and one like it with rear drum brakes. Unless you have a reason to need rear disc brakes, you're going to disappointed in the lack of noticeable improvement after going through all that work. I've watched people do the conversions and almost everyone has ended up with a system that performs much less effectively than the factory-installed systems.

There is something to say for switching to FRONT disc brakes on older high performance cars that are actually driven hard enough to get the brakes really hot. You won't notice any improvement though on the typical family sedan. Drum brakes will lock up the wheels just as easily as disc brakes. The rest is up to the tires to stop the car. You're better off spending your money on four new tires with a higher traction rating. The downside there is faster tread wear due to the softer rubber compound. Here again all four tires should be the same so the front-to-rear braking balance isn't upset.