99 VOLVO VN high gear vbration

i ran a newer model Volvo for awhile and it did the very same thing......the leveling rod was broken at the bottom attachment point on the suspension. you couldn't tell it was broken with just a cursory inspection but to reach down and touch it it was broken clean at the attachment point. as long as the drivetrain was bound up under torque load it was fine but as soon as the torque came off it sounded and felt like the trans was about to fall out in the road.

shop put a new leveler on it and it was like brand new. they said by not allowing the leveler to adjust the suspension to the load it had the drivetrain out of line and that's what was causing the noise and vibration and it was coming from the rear of the trans from being out of line with the rest of the drivetrain.

not sure about an older one like yours but that's what happened with mine.

 

Drive it with out the trailer & see If it Does it again
Then

Check your Steer Axel ,tires , shocks springs alignment....and so on if all good
Then

Take it to a trany Shop { DO NOT GO TO A DEALER Or ONE EYED BILLY BOB THAT WORKS 4 A TRUCK STOP } They are CLUE LESS....
And it will cost too much to take a chance on a maybe...

Get an oil sample for all trany and both rears to see which one is fixing to go out

Have them check the U Joints fly wheel etc.
Good luck...