I have owned several 2.8's in 2WD S-10 pickups, -and in the 2WD light trucks, the 2.8's proved to do just fine. Both of my 2.8 2WD's had in excess of 250,000 miles on them with no major issues to speak of. Granted, I take care of my rigs with regular oil changes, -but I also run the living daylights out of them too. My 2.8's would regularly see 5000, -even 6000 RPM's on the tach (because you have to wring the things out to get any power out of them).
I have also driven many S-10 blazers with the 2.8's, and they're all turds. The 2.8 simply doesn't have enough oomph to push around the extra weight of the 4WD's. In the 2WD's, they seem to do just fine, -but not in the 4WD's.
I would not personally invest the time to do that sort of swap. For what you'll get in return, I think you'd be very disappointed.
Like the other guys have said, the 4.0/AW4 setup is leaps and bounds ahead of ANY 2.8/T-5 setup, -particularly in a 4X4.
I would personally look around for another XJ donor rig that has been in an accident or something, -then nab the parts off of it.
As for the 350 swap, -me personally, -I wouldn't do it. If ya want a heavy V-8 rig, -just go and by a K5 blazer, early ford bronco, -or a grand cherokee.
The 4.0 in my opinion is where it's at......, and besides, the XJ being so lightweight is what contributes greatly to it's off road capabilities. The 4.0 fuel/ignition setup does leave a little to be desired, but on the other hand, they are pretty dang reliable, -and they are very inexpensive in comparison to other rigs of equal capabilities off road. I don't care who's running gear you have, -they will all have their problems, chevy, ford, dodge, toyota, etc, -it doesn't matter, they all break.
With the wealth of information on this forum on the 4.0 and the renix/chryco systems, I'm not one bit scared of keeping my 4.0
My .02