the trailing arm is the same for 123 and 126.

As for a 4 wheel alignment- on a 123 at least- it is not possible to change the rear alignment aside from using eccentric bushings on the Trailing arm(how??), or different springs/shims- so save your money.

and I have done both jobs- in my opinion, I would rather do a wheel bearing in 90 minutes than the 3 days it took to change the trailing arms. plus, if you want to do it right, you need to replace ALL the rubber back there- which adds up in a hurry- 2 subframe mounts, 1 diff mount, 4 trailing arm bushings, may as well do the shocks, maybe the spring pads, you need to remove and replace the P brake cable, as it goes thru a mount on the trailing arm, and then when it is all said and done, the wheel bearing may already have 400K on it.

but it is a DIY that can be done without the(less than $100) special wheel bearing tools.