Nancy-- I live on a hillside in the glacial moraine of Ohio. My typical section of yard is somewhere between 3/12 and 4/12 slope or 25 to 33%, which translates into 12 to 15 degrees. If you truly have 20 degrees of slope, you can still carry dirt uphill with the loader, and down. Keep the loader bucket close to the ground and keep it uphill at all times. The weight of the loader and load will keep the front end down while it's uphill from the tractor. If you can just run straight up and straight down (in reverse) to get to your various gardens, you should be fine.
I think something like a BX series Kubota with loaded tires and/or wheel weights and the smallest loader they sell for it should be pretty safe. Tell the dealer about the hillside, ask him to come out and drive the tractor on it and give you his advice about it.
There are articulated tractors made around here that work well for many. One is the Steiner, another is the VenTrac. The loaders for these machines will only lift a short distance so you can't pick things up high enough to significantly increase your rollover chances. They are 4 wheel drive, low slung, can be equipped with dual or triple wheels at each corner. I don't have an address or email, but maybe you can google them.
Another option is a type of tractor where the axles pivot with respect to the engine/transmission, which remains vertical at all times regardless of terrain. I don't know the name, but I'm sure someone here does. These are designed specifically for hilly terrain, made in Europe I think.
Can you post some pictures of your ground and gardens, maybe with a level line visible?
