J, I can get you a explorer V8 with a tranny for right at 500, maybe a little lower (depends on my local yard's prices for complete motor, which is around 300 ish) and we can pull D over to my shop for the engine swap part of it. I can have it done in about 2-3 days, running. The other options, if you want to keep D over there (and if you have the time), is to get either a 4.0 or 3.0. I can get either motor cheaper than the V8, which is just under 200 complete with entire FEAD, engine harness, and ecu. Do you have a trans for either the 4.0 or the 3.0? This would be the deciding factor. If you don't have a trans for either, I'd say go with the 4.0, only cause they're more of them in the junkyards, so getting them, and parts for them(lets face it, parts for the 3.0, especially trannies, are getting harder to come by) is much easier. If 4x4 is not really a necessity, you can run a 2wd trans which should be even cheaper to run. Keep in mind, this is temporary while you are saving pennies here and there to get the engine setup you want. So, if you have a 2wd trans, you could run that, and either look for a 4wd trans to swap it out in a day(basically, use what you have for now), or just run 2wd til the 4BT swap. Also, are you planning to run a manual or auto trans with the 4BT? If manual, I suggest the SBF ZF-5spd, as it will bolt up to the 4BT(with the correct block adapter), and I might be able to source you one from my local yard for about $150...
SVT