Two options come to mind.
Option #1:
Re-gear existing diff's to 4.56 and add at least a rear locker. Detroit locker is about $500, gear sets seem to run about $250 for each end plus having them installed might run another $200-$400.
The downside to that plan is you'll still have a somewhat failure prone 10 bolt rear end...although with a full carrier Detroit it should withstand more.
Total cost is prolly gonna be about $1500.
Option #2:
Swap to 3/4 ton diff's. 14 bolt FF rear and 8 lug 10 bolt front. These diff's commonly come with 4.10's and can usually be found for under $400 for the pair. Yes, the ratio isn't quite as deep but it would still be a very good improvement over what you have now. Factor in 4 15 x 8 8 lug steel rims and you'll be able to re-use your existing tires...shouldn't run more than $250 for the rims.
Total cost is gonna be about $650. Add a Detroit for another $380 and install it yourself (easy job). So, for just over $1K you would have a bulletproof rear diff with a detroit locker and much better gears.
This is a 'J' code intake:
You can see it's pretty much open from the top except for the divider mine has. Some versions don't have the divider. If you remove your air cleaner assembly you'll see the difference. I don't have a 'C' code intake pic handy...but basically there is an EGR crowding where the air goes in and it's pretty restrictive. The more air a diesel can get the more fuel it can use and the more power it'll make.
Rene