ZJ disc swap IS quick and easy, and with 34" tires thats what your going to need.
Sure, you can mess with the prop valve, change out the brake booster, ditch abs if you have it to just give it brakes without the pump. But at the end of the day your still over tired for a stock setup.
The brake light usually comes on with a loss of fluid or low fluid. Have you checked all the calipers/ master cyliders for leaks? have you checked your fluid level? have you bled the brakes or changed the fluid in the last say... ever?
for what its worth, the disc brake swap is easy as hell. If you have to go through the rear end and replace parts, like drums/shoes/springs and master cylinders id consider this...
My swap cost me just over $200. I used junk yard backing plates and calipers, then bought brand new rotors, pads and ebrake spring kit for I think $225 total cost.
But, it just sounds like poor maintenance and too much tire