Without a noticeable amount of play in the driveline, that may not be your issue, though the loud ting does indicate the u-joints are worn and should be replaced.

No need to put it in 4wd, the front driveline wont be causing any vibrations at speed, unless you have the hubs locked.  Are your front hubs engaged?  If they are engaged, that could cause vibrations, though that is usually when you have an auto-locker in the front.  My '91 with a detroit in the front does something similar to what you are experiencing, but an open diff shouldn't.

If you can, put the rear on a floor jack or jack stands, and rotate the wheels and see if you can hear or see any unusual movement.  If nothing noticeable, with the rear axle on jack stands, have someone start it up and put it in gear, have them run it up to 2nd/3rd gear and see what you hear or see.

Also take a good look at the tires, a tread separation can cause vibrations and noise, often not going away at certain speeds like an out of balance tire can.

I definitely hear a loud “ting” when I first take off in forward or reverse

Hey guys can I swap a rear TRD E-Locker 8” dif into a landcruiser 80 series front diff non locked axle?
If so What’s the modifications and changes, necessary do you still need reverse cut gears and does the LC axle housing need to be notched and tapped?

Basically, just crawl under there and see how much play there is in the driveline.  If any up-dow, side-to-side, means they need to be replaced.  The only movement should be when twisting it, and that amount should be slight, as it is just the slack in the trans gear.

Sound's like it is the driveline or a bearing issue in the rear axle.  If you can put the rear up on a jack, and spin the tires, and see what you hear/see.  Also, see what amount of play you have in the driveline, shouldn't be any up or down movement, and 9nly slight when turning it, about an 1/8".

Before the vibes started, had you noticed a ting sound, like a wrench smacking a pipe, when taking off, backing up.  The sound is an early warning that the u-joints are starting to go.