Truck is a 95 2500 4x4 5speed cummins

Since I bought the truck a few months ago if I'm sitting at a light at idle my ABS and brake lights come on and once I start driving they both go off, I just got used to it.
Today I was on the highway cruising at probably 60-65 and I noticed a slight shake that was never there before, and then looked down and saw my brake light was on. I went to get off the next exit, downshifted mostly but when ever I went to brake I get a really bad shake/pulsation.
Now I drove it about 30 minutes more after that. And I've noticed while braking if I start braking at anything over 25 I'll get the shake/pulsation, but if I down shift and start braking at say 20 I stop normally, no problem. And driving it normal no shaking, but if I'm cruising at 65 and start to slow down (no brakes or downshift, just letting off accelerator) I'll get the shaking again and it won't go away for a few minutes. Any ideas on where I should start looking at?

 

Check all steering parts starting at the column.

Inspect brakes. Make sure you don't have a caliper hanging up. Also check the rear lines for leaks. (including wheel cylinders.)