Yep, I believe you'll have to shorten DS for a 8.25" too.

Lou

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Home of Slant6-powered fun machines since 1988

Yep. When I did the swap in my old Dakota (it oughtta STILL be mine... no $$$$ from the lazy brother in law almost 2 years after I "sold" it ).. another story...... anyway when HE blew up the 7-1/4 and I put the 8-1/4 in it I had to have the driveshaft cut either 1-1/4 or 1-1/2." Measure from the cover flange on back of the diff to the flat face on the yoke where the straps bolt to on both rear ends; the difference you get here is the amount that the shaft will need cut. You DO have a slip yoke, so if you cut a LITTLE more (Not a place to go "nuts") the slip yoke will make up the difference.
Too long, though, will be trouble. The slip yoke needs room to do its job, when you load the trunk or go over bumps,etc, when the springs flex. If it bottoms out on the trans output shaft you can do alot of hurt! I have found that sometimes a shaft from a shortbed truck or a Diplomat/5th Ave will go right in there. Go junking, and take a tape measure. just be sure that you don't get one that has been smacked with a forklift or been laying out and driven over,etc. May be cheaper than having yours cut. But then again having yours cut, you know already that yours is good, just too long. When I did that dakota last year it cost me $48 to have that shaft cut.