I'm no expert with ABS brakes but have rebuilt dozens of older brake systems...There could be many reasons for the shuddering in your brakes but recommend you start with basics:
Get yourself a Haynes manual and read the brake section.
1 Get the wheels off and look at how much of the lining in the pads is left - if it's less than about 3/16 inch or the pad is at an angle due to uneven wear, price out a replacement set(both sides of one axle). The inner and outer pads in each caliper should have similar, but not identical wear. Make sure the bloody truck doesn't fall on you by putting BIG chunks of wood under the axles. Don't trust the jack alone or you'll fu**ing die!!!
2 Are the discs smooth or deeply scored? Is there any contamination visible, like oil, on the friction surface? The disc will probably have a shiny appearance due to 'glaze' - if you have a small circular sander or buffer, use #250 emery paper to ruffle the surface. Wear a mask when you do this and don't use compressed air, just wipe it clean afterwards. Warning, only have the disc rotors 'machined' if they are very deeply scored. Some brake shops take off so much disc material as to render the disc thickness right down to the minimum.
3 Bleeding the brakes should be done every 2 years - use Dot 4 brake fluid. This is not difficult but you need a friend to pump the brake pedal while you control each wheel cylinder. Go on the web, read some of the brake self-help sites before you start and remember, LIVES DEPEND ON YOUR BRAKES! If you are at all concerned about your ability/experience, find a brake shop or competent friend that will let you watch while they work. Learn from them.
