My 4 wheel drive is grinding when trying to engage.

A grinding noise when you put the motorcycle into gear and it doesn't move could indicate several issues with the final drive on your Suzuki Intruder. Some possible causes are:

1. Worn or damaged clutch: The clutch may not be engaging properly, preventing power from reaching the rear wheel. A worn or damaged clutch can cause a grinding noise when you try to shift into gear.
2. Faulty gear: The gears inside the final drive may be worn or damaged, preventing them from meshing together properly. This can cause a grinding noise when you try to shift into gear.
3. Low or contaminated gear oil: Low or contaminated gear oil can cause excessive wear on the gears inside the final drive, leading to a grinding noise and reduced performance.
4. Failed final drive: A failed final drive can cause a grinding noise and prevent power from reaching the rear wheel.

If you are not familiar with motorcycle repair, it is recommended to take the bike to a qualified mechanic to diagnose and fix the issue.

Could be the axle trying to engage or it could be in the transfer case. Suggest you put it on stands all around and have some one listen underneath for exact location while engaging it.

check cv/"U" joints in axles and drive lines If you are on hard surface do not engage 4wd as it creates torque wind up in the transmission and diffs and drive lines. It is to only be used on slippery surfaces that allow the wheels to even put the torque wind up by slipping on the surface

Check the oil in the transfer case. If it has lots of metal in it... time for a new one. They aren't the world's strongest.
Another check: Lift the front of the truck, while keeping the rear wheels on the ground. Engage the 4x4 switch (then turn the truck off) crawl under and try to turn the front driveshaft by hand. If it turns and the wheels turn, the transfer case is the problem if it turns but the front wheels don't... front diff issues. Any grinding, crunching etc is also a clue.

You probably have a front drive splined disconnect assembly that has gone bad. This assembly engages the front differential when 4WD is selected. Below is a figure of the item, it is on the passenger side of the differential area.

Could be bad or sticking hubs, wheel bearings or u joints. When you said it went away when 4wd engages tells me that your right its partially engaged. Had the same issue with my 95 SUV 2yrs ago and had to replace the locking hub assembly. It had the electronic auto hubs. They were shot and starting to fall apart.

check to see if you hear any grinding noise in the front wheel, if there is then one of the actuator in the front wheels is bad. the dealer sells for around 120.

Try removing the front drive shaft from the transfer case to the front differential. With the vehicle on 4 jack stands and the front drive shaft removed attempt to place the vehicle into 4 WD. You may have to start the engine if the vehicle uses a vacuum actuator module to engage the front wheels. When you get an indication that the vehicle is in 4WD try turning the front wheels by hand and see it you hear the same grinding noise .... engage the hubs and try again. Usually if the transfer case is ok then the problem is in the front differential or one of the many universal joints that connect all of the elements together. Check the U joints on the front drive shaft for play as well as the U joints on each of the front wheels.

Most likely wheel bearing or cv joint issue. look for a ripped open cv boot. I believe that year used cv joints instead of regular u-joints. Hope I helped. Thanks

It does not seem that sufficient diagnosis was done. No, you cant give a specific diagnosis like that without tearing unit down. It's likely that they told you the most common failure and left it at that. Likely there is a problem with your unit...I think this becomes more of a trust issue than a mechanical one. A second opinion from an independent reputable shop may help but either way, you need to have it fixed. In not using it much, it makes me wonder how it got worn.
good luck