If nothing else, I'd look into swapping a 3.1/3.4 top end in - even if it's just the intake and support electronics.
The Rochester Feedbacks are a recurring nightmare for me (along with the old Carter Thermoquads, the Ford Variable Venturis, and the Honda/Keihin 3-barrel PITA...) and any fuel injection setup would help.
You've got a couple options, and if you're just swapping the top end, you've opened up your donor pool considerably.
Early FI V6-60 engines used a throttle body (TBI) setup that is easy to service, and you might not even need to get the whole in take - just the throttle body, electrics, and fuel source (pump and/or tank entire.) Getting rid of the feedback circuit in the carb, as well as the increased control over fuel metering you can get with the TBI, is already a plus. Even if this doesn't increase power, it will increase economy - and reliability. Cost? Depends on the option you take and the prices you can expect from junkyards. Donors? Anything with a 2.8/3.1/3.4 with FI. The decision to make here is where to stop - add the TB only for TBI, swap the intake for MPFI, or take the intake and heads to have something that will breathe better as well? Depends on what you can find and what you can save up for.
Next, there's the V6-60 swap - and that's going to limit you some on donors. You'll need to find either a RWD 3.1/3.4 (S-10/S15 or F-body,) or a GM Performance 3.1/3.4 (easily spotted - it has starter mounts on both sides.) The dedicated FWD plant just won't work - the starter mount is on the wrong side. You can quickly check the FWD plant for the second starter mount - look for a copy of the starter pad that's on the front to be on the back of the engine as well. If it's got the pad on the back, it's a usable engine. Again, it's a good idea to grab the electronics as well - you can probably get away with using a later XJ fuel tank setup (for either AMC2.5I4 or AMC4.0I6.) Cost? Whatever your local salvage wants (and probably better than here.) Donors? S-10/S-15 truck, Camaro, Firebird.
If you want to get nuts, you can swap in a small block V8 - either carburetted or injected. I believe both Advance Adapters and Novak Adapters make "drop-in" mount kits for the SBC, which eliminates a lot of fab work. Expect this on to be spendy - not just for the mounts, but for the mill and the new transmission you're going to need (THM700R4 isn't a bad choice, but I'd take a Muncie toploader four-speed if I could find one. I just happen to prefer manuals.) The new transmission should come from a 4WD, and you might be able to find one with a transfer case already on it (Blazer/Pickup/some Astro vans.) You'll want a new driveshaft as well with this one - probably front and rear. Grab the driveshafts from your donor, and salvage the connections (rear slip yoke and whatever the front happens to hook up with.) Cost? Likely to be a lot.
Always remember this - "Speed is a question of money. How fast can you afford to go?" Even if you're not building for speed, building for torque follows a similar maxim. "Power is a question of money. How much can you afford to make?"
With all this in mind, I'd highly suggest you kick open a savings account somewhere, take your "mod money" and put it in there for a while, and allow it to accumulate. You'll proably find that it will pile up fairly quickly, and that will open your options.
All the way around, the very best thing you can do is to remove the 2.8 and be quit of the thing - it's best used as a cement form for an anchor. The revisions applied in the 3.1/3.4 made the engine viable, but the 2.8 didn't serve well for trucks (made power in the wrong place, and not enough of it.)
I'd also be willing to bet that any of the swaps I gave you the "short form" on will be cheaper than the AMC242 swap everyone's telling you to do. Not that there's anything wrong with the 4.0, but if you've got the 2.8, you've got either a 1984-1986XJ or 1986MJ, and that means the 4.0 just ain't an easy fit. When the 4.0 was released for the 1987MY, the front clip/nose was lengthened on both vehicles to accommodate the much longer block. Even the SBChevvy swap will be easier (after all, the 4.0 will also want a different gearbox/transfercase/driveline setup as well!)
Is all of this any help to you?
5-90
Just don't use the stock Jeep carb. It can choke the life out of any engine. And make sure your cat is flowing good. 
