When I firmly press the brakes at say 10mph everything stops great. But I feel a shifting somewhere under the driver seat and towards the the fender well. It shifts again when I take off.

Is this a clue to the ultimate demise of my Rockers???

run i would jack it up with jack stands under both the axles , start it up and put your foot firmly on the brake and then put it into drive let the brake out and slightly press the accelerator and then press the brake again and try to feel if it is still shifting , you could have a buddy look around down there while doing this to see if anything seems irregular. check your motor mounts and trans mounts too as well as spring perches . but really its hard to tell with out driving the vehicle 1st hand

i'd check the cab mounts could be rusted or broken or the rubber mounts have rotted to nothing causing the cab to shift

Motor is tight. I could be the cab mounts. I have not done the fronts yet.

oh i saw one of your posts about body mounts i thought you did all of them so i ruled those out .

HOld your foot hard on the brake while the truck is in gear, then rev it enough to torque the engine and trans. Do this a couple of times and the problem will become obvious when the thing comes apart!hehehehehe--Don't do this in a garage or while parked behind your local police car. I suspect motor mounts, trans mount or spring mount issue. Was a common problem way back when.

Thanks guys. Good advice huck.

Sounds a lot like mine at one point - a bad trans mount that eventially led to a loose crossmember

Check your upper control arm shims. I have seen many of come out and have that exact problem