I believe it would be a problem with the ABS. Larger lighter 20's can be easier to lock up as they have higher pressures and smaller contact patches. What you are feeling is the ABS system trying to do its job. Oh and I am assuming the professional who put the 20's on your ride did get the right offsets and the tires are not rubbing on the wells when the truck dives forward.
i dont agree with above. i have been a tire tech for 8 years in may. 20'' wheels are much heavier than stock. we see this problem a lot. most of the time it is brakes being warped. i know you said you replaced them, but they are not all perfect when bought. when we install brakes we turn the new rotors as well as the old when we do a brake job. you would be surprised how many new rotors are not true and needed to be turned on the lathe. if that is not the case i would have front end checked by an alignment tech. they will usually do this for a small fee if not for free. bad front end parts can make good shake. trust me the offset off the wheel has nothing to do with a shake. it can cause tire rub,but not a shake. i make it a point to know these things in my line of work. i am not trying to be a butt hole, i just know my way around after market wheels and tires.
It must be the rear brakes at fault.
Rotors out of round - need to be re-surfaced or replaced
Your rotors are warped. You need new brake pads and the rotors turned at a machine shop or replaced.
Warped rotors
most likely warped rotors,have em turned or replaced
Rotors out of round - should be resurfaced or replaced
You probably have warped brake rotors. You'll need to have them replaced or resurfaced.
You probably have rusty/scored brake rotors.
the brake rotors are warped. resurface or replace rotors. if the vehicle shakes without applying the brakes, check tire balance.
You have a warped rotor. Remove the rotors and have them turned at a machine shop or replaced if it it too far gone to turned. Replace the brake pads at the same time.
If the car shakes when you press on the brakes, you probably have warpped brake rotors, and the squeak is probably some worn out front brake pads. I would have your rotors checked and either replaced (which Mercedes highly recommends or re-surfaced)
Your brake rotors are warped.