Hey, I don't know if this has been found yet, but I discovered a new glitch while dicking around. This could help you get to North Yankton or get the jetpack or find the meaning of life.
NOTE: THIS WILL ONLY WORK FOR MICHEAL, I HAVENT TESTED IT WITH ANYONE ELSE
WARNING: THIS TAKES FOR EVER TO DO AND IT CAN GET f*ckING FRUSTRATING [I SPENT 20 MINUTES ON THIS PIECE OF CRAP GLITCH AND I STILL DIDN'T GET RESULTS. IF YOU'RE LUCKY YOU CAN GET IT FIRST TRY (LIKE I DID A WHILE AGO, BUT FOR SOME PIECE OF sh*t REASON I CANT REPLICATE IT.) ]
Now that that's out of the way, these are the steps.
1.Buy the garage for Micheal (The 6 car one, costs around 30k)
2.Climb ontop of the tire rack: http://imgur.com/6OTMRhg
3.Get Micheal to trip on the tire by jumping and sh*t.
4. If you do it right, and he trips on the tire infront of him (the on in the picture), You will be teleported under the map.
NOTE: The game thinks you're still in the garage, so if you get in a car, your ass will be teleported back to the garage.