The "back door" is a popular route that a lot of people staying west of WDW use.
Its very easy and can save up to 35 minutes at rush hour. Simply proceed up to the 192 (using US27 which is the road that Highlands Reserve is located off of). You will head east on the 192...after about 2 miles you will notice a large Country Club on your left - this is the Orange Lake Resort. After you drive past this look out for "Black Lake Road" which is the next traffic light after you have cleared Orange Lake. It is sign posted just before you reach it. Make a left turn here and follow it to the end. It ends at a junction with "Sherberth Rd" and you need to turn left again. Follow Sherberth to the end, where you will come out at a traffic light. Turn left for Animal Kingdom LODGE only (straight on is WDW buses ONLY) and all other routes are a right turn. You are now on WDW property and the Osceola Parkway which will take you right through to the I-4 at Lake Buena Vista. Follow the signs for any of the WDW parks and resorts. The parking lot to Animal Kingdom is a sharp left only a few hundred yards from the Sherberth Rd lights, so keep left if you are headed there!
To do this route in reverse from the I-4, you need Exit 65b (signposted Gaylord Palms and Osceola Parkway) then follow the signs to Animal Kingdom. Remember to turn left as you approach the park (look for Animal Kingdom Lodge) or you will end up in the parking lot!