I just put two pairs of 235/75x15s on my 4x4 Tracker.  They were both used (operating in severe penny piching mode here).  Even though they are stamped the same and look the same size, when I measure them the rears are about 1/4" taller.  Is that enough to cause any problems in 4wd?  I will only use 4wd in the dirt or on snowy roads (if we ever get any down here),  but its possible I would traverse some pavement in between snow/ice patches.  I'm thinking 1/4" isn't enough to worry about on a 28/29 inch tire, but I thought I'd ask anyway. 

I think you will be fine as long as there are 2 of the same tires on one axle.

Ever since I spun out on a snow-packed Colorado mountain road, doing about 30 mph with 4WD engaged several years ago, I have been very leary of using 4WD at road speeds.  With the smaller tires in front, enough difference in front-to-rear tire diameter can cause problems if you hit an icy patch, especially if you are turning even slightly.  I don't know how much diameter difference it takes to cause problems.  Even if your tires are matched, differences in air pressure can give the same effect.  My gut feel is that you are probably ok.  I would just be especially careful on icy roads.