I can add some first-hand insight here.
91 had a 2.25" diameter pipe all the way to the rear bumper.
In 94 they reduced the diameter on the backside of the muffler.
In other words...any STOCK muffler after 94 will be a 2.25 IN and a 2" OUT.
Really messed up.
Also the 94-newer tailpipes have a bend at the tip-end whereas the 91 DOESN'T and the lead-in pipe before the axle "hump" is longer on the 91.
They say a picture is worth...well you know.
If you are looking to replace the muffler and tailpipe. This is a good mod.
I have BOTH a 91 Laredo AND a 94 Sport.
My 94 needed an exhaust system...
I purchased a Dynomax Super Turbo (forget the number but if you use the "WALKER MUFFLER" exhaust system diagram for a 94...its there.)
It had the 2.25 IN/CENTER and 2.25 OUT/OFFSET
Then I purchased a 91 tailpipe.
Just install everything as you would normally. Only modification is the front of the tailpipe needs to be shortened.
Stock hangers will work just as they would normally.