Have a seasonal 89 Volvo (think they call it white GMC on title) live tandem with a locking differential.

Was driven for about a month at lower speeds probably didn't get above 40, no issue. Sits a few months then driven at higher speeds and past 40mph developed a terrible vibration. This is with a trailer.

One of the ujoints felt a bit sloppy so changed it. Test drive without trailer and still vibrates. Tried lowering suspension which I mean let out air bags over rear axle and... vibration goes away.

Any ideas appreciated.