Pingpong has an XJ with a 4.3 in it.
Hard swap? Wouldn't say so, just a lot of stuff that has to be done. Is it a TBI engine, or vortec, or carbed? I'd not bother with a carbed setup, unless you ran propane, but that's a hole 'nother bridge.
You'll need some funky headers, or the stock cast exhaust manifolds. You'll need to adress the fuel system (depending on type). You'll need another tranny - whatever you have now will probably not be up to a 4.3 on way or another. The one in the van may or may not be useable.
Then there's cooling. A 4.0 rad would get the job done. And you'll need to change your fan setup - I'd suggest electric.
You can buy motor mounts from AA. I'd probably not.