Disc Brake Axle from 2003 into 2001 Question
 

I changed the axle on my 2001 SLT 4x4 from a 2003 because it had disc brakes and a 3.92 gear. So I swapped my LSD into the new 2003 axle and installed it. I see that my 2001 had a pressure limiting valve (proportioning valve?) and the 2003 does not. The part number for is 5003312AA. Also, the ABS controllers are different #'s. My question is, do I leave the valve it or remove it, or is there another option?

I haven't done this myself on my Durango, but generally discs need more pressure to work due to the self engagement action of drums

Yes disc brakes will require more fluid. More amount of fluid is needed to push the bigger piston. I did nothing with my proportioning valve when I put the 03 rear end in mine. Mine actually stops better than before I swapped, but there are some terms to that. 1. I had gear oil soaked into every crack and crevice of my brake drums/shoes before I made the swap. So they were never working up to par anyway. 2. I've had a spongy pedal on my Durango for years. What I've noticed recently though is that when it gets colder and more humid, my pedal feels better and the brakes are a little more balanced. Not sure why. But yes, generally speaking, you should need more fluid for disc than drum.

So it almost sounds like the swap wasn't worth it?

I believe also from everything I have read, the 2003 also had larger rotors and calipers up front. I have a 2001 and have been toying with doing this. I wonder if just adding the bigger caliper and rotor upfront would work with changing out the master cylinder?

Part numbers for the master cylinder appear the same for 99 and 03.

I wouldn't bother. Actually, I didn't bother. I saw one in the junk yard on my last trip just waiting for an easy pull. I thought 2 seconds and decided I'd be fixing a problem I don't have. My '01 R/T brakes fine, whether pulling 5500 lbs or not.
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I swapped the front last spring to the larger and better designed '03 brakes. It was definitely worth it. my rotors wrapped quickly iwht the original setup but it doesn't do it anymore. The '03 uses a traditional slide pin like most other disc setups instead of the awful "ears" that the 98-02's used. My original setup had digits in the "ears" where the pads rode probably causing rubbing and then wrapping. I recommend the swap if you have it available. I would do it when you plan to do your brakes again because the parts cost the same between the 2 setups. I also did my upper and lower control arms, tie rods, and steering rack as it was all due for replacement (250k)

I don't mind the drum brakes but when I was hunting 3.92 gears, the axle they were in was an '03 so I thought the disc brakes were a bonus. it has been a lot of work to swap everything out but I chose to rebuild the trac-loc while I was there, seals, ...... I haven't driven it yet because I still need to install the new front axle with the 3.92 gears.

I plan to test it with the prop valve in place and then try unplugging it to see if it makes a difference.

Well it was worth it. My original from 98 had run so low on gear oil that the internals were trashed. So I had to get a new rear end regardless. My problem now is the spongy pedal that my truck has had for years. This makes absolutely no sense I know that, but believe it or not my spongy pedal started when I did a torsion/shackle lift. Never cracked a brake line, never changed pads or shoes. Made absolutely no sense whatsoever, but that's what happened. Drove it down the street and immediately noticed more pedal travel required to stop the truck and ever since then it's like it sends more power to the front than it should. And that was with the original drums in the rear.

But yes, I've heard the master cylinders never changed all the way through.

Can upgrading the front to 03 style just be as simple as getting new calipers, brake pads and rotors? Or are there other hardware things that need to happen further back?

Nope, the 98-02 knuckles have a cast "shoulder" that the pads ride on. The '03 doesn't have this and instead uses a more traditional caliper mounting bracket and slide pins. you must have the '03 knuckle to make it work. you can however reuse your hubs/wheel bearings.