I have a 2002 Yamaha kodiak .

The 4 wheel drive quit. How do you replace the 4 wheel drive motor?

Also, when I drive down the road, the machine feels like it lifts up a bit and the goes back down. Could this be the 4 wheel drive engaging and disengaging?

 

Could be, firstly are your front tyres a match for the rears? If the machine is stuck in 4WD and the tyre diameters don't match, the different distance the wheels move per rev has to come out somewhere. On soft ground it can skid a little, on concrete/asphalt it winds the prop shafts and drive shafts up like torsion bars then the tyres slip or something lets go in the drive and you feel a bump. Had a Honda 300 that did this big style, on checking, the chassis had rotted through just behind the front lower A arms (common on old 300s), the frame was springing apart when the tyres wound the transmission and frame up, this pulled the front prop shaft out of the diff splines and let the tension out with a bump.

As for replacing the 4WD motor, there is a long post on here about it from someone selling 4WD motors (Yam make you buy motor and gears as a unit). He got Trolled by someone taking exception to him making money out of selling motors, so may have given up.

As well as dodgy motors, the front prop shaft splines, where it goes into the engine, are prone to wear. Lift the front on a jack, stick it in 4WD (ignition on) and spin the front wheels by hand, if the front prop shaft turns it is faulty splines. However if the drive is locked, the shaft is OK and 4WD is engaged, so switch out of 4WD and if the wheels spin free the 4WD motor is working.

 

Here is some good videos on it. 501parts.com - Yahama Servo Motors