08-01-17, 01:38 AM
I'm sure a decent shock will be an improvement even if you don't find anything wrong with the linkage, but it's possible that temperature has a lot to do with it.
The first 8 miles of my commute to work is on slow twisty roads with plenty of undulations, pot holes and long shallow depressions where trenches dug for pipes have sunk after repair. It feels similar to your description: a jarring ride and I keep thinking the back end's trying to step out.
OK, I've a knackered shock and the rear tyre's beginning to square off, but in winter there's a huge difference in how the bike feels in the morning when the tyres (and shock oil) are cold and when I come back at night after a long blast down the M4 and they've had a chance to warm up.