4.2 in an xj??

well you know what i'm saying i want to take a 4.2 block and put in in my xj with my current 4.0 head seems like the eaziest way to slap some power in without a total motor tear down and rebuild and searching for all the parts for a stroker i would use the existing efi and computer if possible. soo for all you motor heads out there tell me what you think and if it will work? how much different are the blocks.

o yea this is all in a 96 with an ax-15 and obd-II

I seem to recall having heard of it done somewhere. (My subtle recommendation to search.) I think it just involved a slight modification to the cooling passages in the 4.2 block.

Swapping in the 4.2 short block will be easy as long as you use everything else from the 4.0 (including the 4.0 head with all the 4.0 ancillaries). You'll need to block off the outer triangular coolant passages in the 4.0 head but that's about it. As long as the 4.2 block is an '86 or later version or an '80 or earlier version, your 1/2 " head bolts will go into the block without any problem. The '81-'85 4.2 engines used 3/8" head bolts.
To mate the 4.2 crank to the '96 AX15's input shaft, you'll need the pilot bearing with sleeve that carries part no. 53009180.

how much of a difference in power do you guys think i will see i was thinking of buying a short block from rebuilt-jeep-engines.com and having the head's off my 92 ported and rebuilt and then slaping them together (after pluging the passages) by the way what is the most popular way to plug them?

You'll gain more torque at lower rpm using the 4.2 short block with stock 4.2 cam at the expense of higher rpm HP. You'll regain some of that HP by porting the 4.0 head so overall you'll have something of a mini-stroker.
I believe J-B Weld works well in plugging the coolant passages. Just go to Tim Weaver's article to find out which ones you need to plug.