does an '85 third member fit in a '81 rear end?

 

yes...but make sure that it is the same gear ratio as your front third...assuming 4wd.

 

What would it do if they were different ratios?

 

If the ratio is out enough, it will cause a bind in your drivetrain and most likely, something is gonna break.

Tye

 

You'll grenade your t-case and/or diffs.

 

say i wanted to put 4.10's in the back, but leave the stock 3.90's in the front? does that grenade it?

 

yes.

Different gear ratios = front and rear wheels spinning at different speeds = not good.

I suppose you could do this, and then put smaller tires on the rear than up front...

 

Are you trying to destroy your truck? If so, you're on the right path.

 

Haha thanks

 

no i just found a cheap third and want a little better gas milage

 

Buy matching front and rear gears. always.

 

you can find 8inch 4.10s for dirt cheap. i think your the only person ive seen that actaully wants one haha.

 

well you can do much with 3.90's except getting past by every car

 

haha, the one year that Toyota did this, sux for you.

If you are gonna re-gear, do it for real. There is no point in saving a few bucks and then having to re-do your work later. If you are thinking you would like to re-gear, try a 4.56 or 4.88. I would go for the 4.88.

 

yah, well the transmission has gotta come first

 

sounds like my yota with 4.10s but seriously look around on the calssifieds here, craiglist, and junkyards and you can find a set of 4.10 diffs

 

yah, i guess i can add it to my junkyard list