99 f250 single rear wheel. What size are the axle tubes?

[wckdpowerstroke]

Im going to put some 2.5 procomp leveling leafs and 35x12.50's on my truck. I also do not want a saggy ***. So when i ordered the front leafs i did ask about getting a bigger block for the back.

My question is what size is the axle tubing on my truck for the u bolts? He told me 2 sizes. I think 3 5/8 or 3 7/8's

Thanks
Mike