I found a used brush guard that was on a 95 sidekick for sale and I'm wondering if it well fit on my 91 tracker. Or are the mountings going to be different? Below is a pic of it. Thanks for any help you can give me.

Are there any frame mounting brackets that come with this brush guard that you are looking at?  It should have some brackets that bolt securely to the frame to attach the guard to.  If it doesn't have any brackets you can fabricate them.  I'm thinking the frame widths didn't change on the trucks.  Measure the outside width on the bottom of the verticals of the brush guard then get under your truck and measure your frame.  If they over-lap (ie: brush guard is 27" outside to outside and your frame is 27" inside to inside) or some variable very close, you should be able to make the brush guard work with a little creativty, some card board to build templates with and some 1/8 plate steel... etc.  If they are selling it for a reasonable price (for not being complete) then go for it.  Reasonable price = worth it to you knowing you will have to fab some frame mounts.  Doubtful you can build a brush guard yourself as nice and I doubt you will find many manufacturers on the market with brush guards today aside from Calmini type variables, etc.  

He is asking $75 OBO   I'll ask if he has the frame mounting brackets for it.

$75 is decent price... $50 would be more better (if the parts aren't all there), BUT really even if all the parts weren't there you would still have a difficult time beating the $75 price by building one of your own (providing the brush guard isn't heavily rusted or bent).  It looked pretty clean in the pic.  A little bit of sanding, clean it and paint it.  It would look good as new.  If it is heavily rusted or has holes rusted through it, I wouldn't pay that price.  That's just my thoughts.  Offer them $50 or $60.  You can always come up.  Once you buy it, it's a done deal.

Bumper mounting for a 91 is the same as a 95.  So if it bolts up to his without mods it should bolt up to yours.

Some models did have slightly different front bumpers, so some bumper trimming may be required.