I am new to the forum and this topic has probably been beat to death.  I apologize ahead of time. 

77 stock L48 

While in traffic (stop and go) I get a shudder.  Temp gauge is only reading 200. I'm thinking tranny overheat/torque converter issues.  

Is it only while you're sitting still, idling? Or as you're moving slowly?

Moving forward slowly.  When I let off the brake it grinds and shudders. In normal driving conditions it feels fine. 

Fluid is fine and a few months ago I dropped the pan and replaced the filter. 

Lotta questions, so here it goes...

1: Does it do the same thing in reverse?

2: Are you certain it is not a brake issue?

3: Does it ONLY do it in 1st gear? 

4: Are you certain it is not a brake issue?

5: Does it only do it going straight, or when turning, or both?

6: Are you certain it is not a brake issue?

7: Are you certain it is not an engine miss?

8: Will it do it with the shifter in manual low?

9: Are you certain it is not a brake issue?

10: Have you checked the rear wheel bearings for play?

1st thought is a brake problem.

2nd thought is possibly the posi clutches binding.

3rd thought is an engine misfire/vacuum leak. Releasing the brake pedal when the booster diaphragm is cracked will cause a fairly large vacuum leak for a few seconds, causing the engine to run rough. 

Lastly, it could be a flywheel issue, causing a vibration, and a grind/popping noise at times, especially at idle in gear..

It could possibly be a planetary issue in the transmission, but not likely. Planet issues will cause noises in 1st and second gears, but not 3rd, and make a lot of noise as well as in reverse.

400 transmissions and/or torque converters are not known for any kind of shudder problems. Did you have this issue BEFORE changing the fluid & filter? If not, did you replace the o-ring seal on the filter extension tube that fits up into the case?

Today it did it more.  I was able to get to a quiet spot and open the door.  It sometimes feels and sounds like it's coming from the tranny.  Today sounded and felt more like rear end. 

I replaced all the u joints but didn't repack the rear bearings or check the diff.    

Things done a few months ago;

New water pump

Flush radiator 

New dual timing chain

Rebuilt front end and 4 new shocks

Rebuilt original Quadrojet

New true dual exhaust

Removed all unnecessary emissions 

Recurved distributor 

New steering box

New brakes and rotors