Follow these steps in order:
1-Once you're certain that the lines are all right and especially that the metal pipes near the back plates have no sharp bends to create restrictions in brake fluid pressure, get hold of a brake fluid pressure testing gauge and test per the manual specs on each corner. This will give you a good idea of the master cylinder/servo assists health as well.
2-Make sure your leafs have the same spring rate and the shackles and U bolts are all in order.
3-With the first two things done, adjust all wheels starting at the farthest from the master cylinder and working your way to the closest. This is how its said in the manual for my FJ70. When adjusted, each wheel should have a slight drag on it and should be able to make two full revolutions when rotated with a good bit of force.
4-If the problem still persists, take a look at your wheel cylinders and ensure that all are in a comparable(workable) state.
5-Check your retaining springs(long/hard springs that hold the brake shoe in place). If any spring is softer, it will be easier for that cylinder to push up the brake shoe when you press the brake creating a greater braking force on that wheel and pulling the vehicle to that side.
6-Font/Rear drum brakes work on different master cylinders as compared to rear only drum brakes with disks up front. The local parts guys here almost always have brake cylinders here that are for Disk/Drum vehicles and they sell them 'jhoonga' style to all types of brake setups. Their only criteria is matching bolt patterns i guess.
Anyway, if you had the master cylinder changed and your rear locks up, i am 100% sure that its because of a Disk/Drum master cylinder that you replaced your original with. The reason is that a Drum/Drum master cylinder has pressure retaining valves where the brake pipes go on the cylinder, that maintain a certain fluid pressure in the system. When you replace this with a disk/drum master cylinder, those valves are only in place where the pipe for the rear brakes goes on. As a result, the rear brakes have good baseline pressure to work off of when you push the brakes where as the front has none. Consequently when you push on the pedal, the rear tends to lock up and the front brakes come into action later on when the pedal stroke has generated sufficient pressure.
Try interchanging the front and rear lines on the master cylinder. If it fixes the rear lock-up issue, you can be certain that the problem is due to the reason described above.
A properly tuned drum brake system with all the right components works just as well at stopping the car under most conditions as a disk set up. Give it a try and revert with your findings, lets see.