Is it feasible to put an 87 Bronco Rear ABS onto an 86 Bronco? If so how hard/expensive could it be?

 

I don't know why you'd want to do this, but you'd need to transfer the modulator valve on the inner driver's framerail. You'd also need to have an 8.8 rear end w/the sending unit plug installed on top, and of course the excitor ring in the diff sandwiched between the carrier and the ring gear. There's also gonna be some associated wiring I'm not famiar with, but the RABS is de-activated in 4WD, so something would be wired for that, plus the ABS light in the dash.
Most people spend their time disabling their RABS, not retro-fitting it.

 

You can have ther ABS system from my 90! Take it, please. Actually I have not seen the ABS light for a while (of course, I took the bulb out!) Brakes work fine, don't know if the ABS works and don't care.

 

This actually crossed my mind, but it would be alot of work. ABS is nice to have, but Ford RABS have it own computer, I know this cuz I once had problems with the RABS on a '89 Bronco II. It ended up being a $160 proportional valve.

If you have all the parts, and know what your doing, I say go for it. All you have to lose is time, right?