Hi Im 17 and just bought my first truck. its a 94 dodge dakota. when i took it for a test drive the 4x4 seemed to be working, today I engaged 4x4, started to turn the wheels and the floor started to vibrate and sounded like something hit the floor. when i drive it in a straight line it doesnt do it or when im in 2 wheel drive. thank you in advance

 

If you were on dry pavement that's probably the reason. Part time 4x4 should only be used on loose or slippery surfaces.

 

the transfer case in most part time systems does not have a differential in it to allow the front axle to rotate at a different rpm than the rear axle.
If you understand that as you round a bend, the outside tires travel a longer path than the inside tires, it is the differential in the axle that permits the tires to do this without skidding on the road surface. The same principle applies for tight turns in 4WD, where the front tires and rear tires also travel a slightly different distance. What you are feeling and hearing is torsional windup in the axles, driveshafts, etc. Just as Festerw advised, using 4WD on high traction surfaces will cause this, and should be avoided to keep your u-joints in one piece.