Yes you can swap front and rear diffs on your 85'.

As far as recommendations on 3rd members, i know the feeling of being low on funds due to being in school.  Check out your local pick and pull yards.  A lot of 3.0l v6 2nd gen runners came with 4.56 gears And the automatics that came with 31" tires had 4.88 gears factory.  The 4.56 diff will make some difference over your 4.10s.  If you go with the factory 4.88 diffs, you need to know they are special in the fact that you cannot regear those diffs later due to the housing being more offset i believe.  You might get lucky and find a couple 4runners with the same gear ratios. With that said, around here at the local junkyard i believe you could get each diff for like $80ish. If you can find two 4runners at the same time That is a pretty cost effective regear.