Can I change my...

Rear drum brakes to disk brakes?

Anything is possible. As long as it was an option for the model year you own. I would almost bet the wheel bearings are different maybe even the rear axle. The brake hoses for sure and possible the steel lines. I would bet the master cylinders are probably the same.

would I be opening a can of worms or am I better off with what I have?
I found these

http://www.am-autoparts.com/AM/Rear-Wheel-Hubs/Chevrolet/Venture/1999/AM-25153097/

I guess you'll have to look at the cost of replacing the calipers, rotors, hoses, possible lines, possibly the axle, E-brake cables, and wheel bearings.

The other thought is to get a used rear axle with all the parts (with any luck in good working order).

Personally I would roll with what you have.

You could...as the rear discs were an option on the '99s, but as stated previously, you would have to weigh the benefit for the costs involved in the changeover. And ask yourself if it's worth it to spend that amount of money when your present braking system is adequate for the vehicle?

To AdVenture,

You and I must think a lot alike. I was wondering the same question when I had to change the brakes on the rear of my van. I also have drums. I researched forever trying to find a disc brake conversion kit and all I could find was aftermarket stuff that probably required some bracket work. I hate drum brakes with a passion. I eventually gave up looking and changed the drum brakes. There was a little cussing involved, not much. It eventually went together. If you find something, please let me know. I would still be interested in changing them down the road. Good luck.