Lowered rear suspension question?

[Todd68Chevy]

I have a 68 with coil spring rear suspension. I lowered it 22 years and 38,000 miles ago by heating the springs front and rear. The truck was 4-4 1/2" lower and worked and drove great. I finally put disc brakes on it last year. When I did I changed to lowered coils and dropped spindles. The front end up approx. 6 1/2" lower than stock and the rear is 5". I was going to install an adjustable track arm but it would not fit because of my exhaust so I figured it worked for all this time maybe I will not need it. Still seems to drive good but when hitting bumps some times it feels funny. I have put approx. 2,000 miles on it since changing everything. Couple weeks ago I took a 600 mile trip to Michigan. A buddy went with me so it was the 2 of us, luggage, coolers, tools, ect. I noticed on the way up there, that every bump felt funny (like the rear end is floating around). Then when we got onto the Interstate it was real bad. There was some of the bumps in the road that felt like the truck was jumping around. After unloading stuff up there it seemed back to normal. Then after loading up to come home it was the same way.

Now for the actual question. I am assuming this problem is because the rear track arm is in a different position and needs to be fixed. If you have a different opinion let me know. What do I need to fix this problem? I have a 5" drop with springs only, so if I modify the exhaust and install the adjustable track arm, will that fix it or does the track arm mount also need to be relocated? I have done a few searches but could not find the answer.

Thanks for your time and knowledge.