Just recently bought the RE 3.5" lift (full leaf) and of course, you have to purchase the U-bolt kit to fit the 8.25 axle, whats the difference in the U-bolts for 8.25 and the Dana's?

Size of the axle tubes. Don't know the specifics though. But i bet it is googleable.

8.25 is 3" wide tubes the dana rear is smaller

you don't have to buy from RE, but you do have to use the correct size bolts. Call around local spring/drivetrain shops, ask if they make U-bolts. you'll need the axle tube diameter, the bolt size (1/2" - 9/16" IIRC--measurewhat you have), and min. bolt lenght (bottom of axle tube to top of spring plate) plus some for washer/nut +1/2" or so.