Which part has been removed -- the front distribution block, or the rear proportioning valve? If the front distribution block has been removed, how did you drive the truck before you took it to the shop? If the mechanic removed it -- tell him to put it back in, and then find a different shop.
If the rear thing has been eliminated, you don't have a proportioning valve (unless the front distribution block has been replaced by an XJ or ZJ front proportioning valve. My advice is to buy an aftermarket adjustable proportioning valve and install that in the line to your rear brakes. That will allow you to tune your brakes to avoid premature rear wheel lock-up (which is the purpose of proportioning valves). However, as I think someone already mentioned, if you tune it with the bed empty, you should readjust it if/when you carry heavy loads. If you tune it with a heavy load, you MUST readjust it when you run empty.
From the factory, the MJ had TWO hard lines from the front to the back. One fed the rear proportioning valve, and that was the primary, everyday brake circuit. The other line was an emergency by-pass line. If the front brake circuit fails, you want full pressure to the rear brakes, so the emergency circuit by-passes the rear proportioning valve to deliver full pressure to the rear brakes.
After re-reading this entire thread, I'm still not clear what was removed from your truck, or what "problem" your mechanic was having. And did HE remove something, or was something removed by a previous owner?