fish-tails right, pulls leaft

My '88 C-4 has a strange problem. I got a clean car from a good friend of mine a long time ago. I don't run it hard so it took me a while to realize it has a problem.
Hit the gas in 1st gear and it goes straight and fast. It's an automatic and it shifts really good. When it hits 2nd gear, it quickly snaps the rear end to the right. It caught me off guard, so it scared me the first time it happened (could have caused a collision). It's not so bad since I can predict it now.

The brakes always felt weak and funny, and the master cylinder started leaking recently. I replaced the master cyl and now when I hit the brakes, the rear end crabs to the side and I have to counter steer to the left.
It's like the left rear wheel/hub has slack in it and it's floping side-to-side, making for a goofy ride. I checked the hub, the joints and the tie rods. Everything is tight. The only normal slack I found is inside the differential yoke clip.

All I can figure is the high torque under power and braking is pulling slack in the left rear tie rods. Could this be correct?

Anyone with experience on this problem would be helpful.

Thanks.

There should be no slack in any tie rods, but that does appear to be the case - at least something in the rear end is causing issues. Check the condition of all of the rubber - it's an '88 and if the rubber is original, perhaps one of the bushings met its maker.

Also check the torque arm at both points on the transmission and on the differential.

If I hold the brake, shift it from reverse to drive/back to rev., I can watch the rear wheel and see it slightly pivot side-to-side.
I'll have to get someone to do the same while I check for spindle movement. It's either a bushing (somewhere) or a hub bearing (no roughness while driving though).

I'll let you know what I eventually find.

Thanks!!

sounds like weak bushings on the rear control arms or torque rods