It may be that the cutlass bearing is shot and causing the shaft vibration in gear
, but it is just as likely, and maybe more likely, that the pillow block bearing in the center of the shaft is worn out. As you probably know the T30 was fitted with a Babbitt pillow block bearing in the center of the shaft to stabilize the long unsupported shaft. The bearing was equipped with a grease cup that was supposed to be turned down a bit every couple of hours when underway to lube the bearing, but most owners forgot all about it until there came a knocking from beneath the floor boards, and by that time the Babbitt bearing was a goner.
The fix that I used on both of the T30s that I had over 20 years was to replace the Babbitt pillow block with the same size ball bearing block. These usually come with a zerk grease fitting, but I replaced that with the original grease cup and used water
pump grease in the cup. Even with non sealed bearings I never had a problem with rust from bilge water
, as the grease does a good job of protecting the bearing. The ball bearing won't ever wear out as the shaft speed is low and the load on the bearing is almost non-existent. Just a twist of the grease cup occasionally will keep it going forever.
Meanwhile, to move the boat in the short term, look into this...
My recollection is that the pillow block bearings were the split type, with a removable cap. If that's the case, remove the cap from the bearing and judiciously use emery cloth on the mating surface between the upper and lower half of the bearing. Put the cap on a sheet of fine emery on a flat surface like a table saw table or a piece of glass and remove a bit of material. Check the clearance on the shaft frequently to avoid removing too much material. This should allow you to tighten up the bearing clearance enough to get where you are going without problems.
In the event that the problem is really the cutlass bearing, ask one of your buddies to give you a tow, because that could be difficult to change in the water.