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I believe that you are on the right path if you are going to replace the U-joints.
Driveline vibrations from the output of the trans up to the pinion gear happen at 3, 4 or 5ish times the speed of a wheel vibration. It is dependent on the final drive gearing. Your drive shaft turns much faster than your wheels do though.
Driveline vibrations often feel fluttery for lack of a better term and are commonly load induced as you describe. They often times come with a droning type of noise or sensation.
As far as inspecting u-joints. They more often then not will feel tight with the wheels on the ground because the weight of the jeep and it being held in park, neutral, whatever...... it really does not matter but the shaft will be preloaded in one direction or the other if the wheels are not in the air and can easily mask any play in the joints. Good joints have no play whatsoever that is discernable to the naked eye. Some bad u-joints have no play in them at all but if you remove the drive shaft and move the joints through their range of motion you will find that they may bind or not want to move at all in a certain direction. Ones like this will certainly vibrate and when they are removed they generally have insufficient lube and galling inside the caps where the needle bearings ride.
When you install new joints, they need to move freely. I personally do not use a press as it can bend the ears of yokes or shafts in somewhat and create a binding condition with a new u-joint. I have an XJ in the shop that I am installing new u-joints tomorrow. I need to remember to take pics and maybe a video of how I do them. I am picky about how to take them out and install them to prevent binding and bending.
Driveline vibrations from the output of the trans up to the pinion gear happen at 3, 4 or 5ish times the speed of a wheel vibration. It is dependent on the final drive gearing. Your drive shaft turns much faster than your wheels do though.
Driveline vibrations often feel fluttery for lack of a better term and are commonly load induced as you describe. They often times come with a droning type of noise or sensation.
As far as inspecting u-joints. They more often then not will feel tight with the wheels on the ground because the weight of the jeep and it being held in park, neutral, whatever...... it really does not matter but the shaft will be preloaded in one direction or the other if the wheels are not in the air and can easily mask any play in the joints. Good joints have no play whatsoever that is discernable to the naked eye. Some bad u-joints have no play in them at all but if you remove the drive shaft and move the joints through their range of motion you will find that they may bind or not want to move at all in a certain direction. Ones like this will certainly vibrate and when they are removed they generally have insufficient lube and galling inside the caps where the needle bearings ride.
When you install new joints, they need to move freely. I personally do not use a press as it can bend the ears of yokes or shafts in somewhat and create a binding condition with a new u-joint. I have an XJ in the shop that I am installing new u-joints tomorrow. I need to remember to take pics and maybe a video of how I do them. I am picky about how to take them out and install them to prevent binding and bending.
