Start off with a zero-cost test (apart from a squirt of fuel).
Vibration is almost always down fo a failed UJ, and as Series vehivcles usually euse the rear axle only, it's almost always a rear prop UJ.
Remove the rear prop, engade 4wd high (yellow knob down), and test drive. If the vibration is gone, it's a rear prop UJ. If the vibration persists, refit the rear prop and try again with the front prop off. Again, diappearance of the vibration will confirm a failed front prop UJ.
Second stage is to check the wheel bearings by gently rotating each wheel by hand, feeling for notchiness, noises and free play. Check the swivel pins aren't worn at the same time.
Next, get the wheels' balance checked. This is fairly cheap. Also have the tyres checked for bulges, flat spots or defoermities.