Originally Posted by Mortalis5509
The lower arms control the wheel base. The upper arms adjust and hold the pinion angle where it is.
This is incorrect and is just one of those internet myths that someone started years ago. Neither set of links has a "duty". They work together to position the axle--adjusting either one will change the pinion angle and position of the axle. If you want to place the axle where it needs to be correctly while also providing the proper pinion angle at ride height, you need both adjustable uppers and lowers.
As for your problem, welcome to the land of rubber bushings and the associated deflection and wear they bring to the table. If you want a much more durable setup that won't allow unwanted movement in that direction, take a look at Currie JJ's. You can get a set of adjustable arms with JJ's at both ends from Savvy.