Long enough to reach between the t-case and the diff, with enough slip joint compression movement available so it doesn't become "solid" (bottom out) when the suspension is fully compressed. If it's stock, a driveline shop should know the length, but if it's lifted, you'll need to give them more info. Measure between the face of each yoke when the truck is sitting on it's wheels and give that dimension to them. Also give them the height to center of each yoke, and they'll probably also need the distance from the bump stops and where they hit when springs are compressed.how long should hte front drive shaft be?