Lot of things it could be. worn/broken mounts, allowing the T/case or trans to move before locking up. Trans problem with delayed engagement, slop in T case chain, worn U joints.
Going from anything to drive makes the torque go from one direction to the other in the drive train.
If you trust your bud, get under the jeep and have him work it through the gears, see if something is flopping around underneath as it torques in different directions.
97 Sahara, 74 Renegade, 72 CJ5, 55 Willy's (the Beast) Boss 302, 6:34 gearing, Bose 901 speakers.
God I miss that Jeep. Forgot the kinda Jeep, 80 SX4.