I don't think it would cost $3000, if you find that parts you need in the right places.
But off the top of my head, here is the parts that you need.
2 - 4wd Spindles. Make sure you get the 2001 to 2005 Sport or Sport Trac spindles. If you got spindles from a regular Explorer you would also need the brake rotors from the regular Explorer.
2 - 4wd Hubs
2 - CV Axles
1 - Front Axle with matching gear ratio to your rear axle.
1 - Front Driveshaft
1 - Rear Driveshaft
1 - Transfercase skidplate
1 - Transfercase. Preferrably a manual 1354 with linkage. If you get the electric 1354, then that complicates things. With the electric one, you will need a GEM module and switch and maybe some other stuff.
And the tailhousing on the transmission would need changing. A transmission shop would need to do that.
You can do it if you are up to the task. Kris Guilbeaus here in Texas has never bought a 4wd Explorer. He's converted several 2wd Explorers to 4wd's. All those were 1st gens, but the principle is the same on the 2nd gens.
There's a vendor on here that is a junkyard in Kansas that specializes in Explorers. ped5stang is their username here. You could price out all those parts from them.