i have a 91 ford f-250 that i want to put a soild axel on cuz my twin i-beam is sagin really bad. i was wondering wat is i good axel to put under it. thanks.  

high pinion dana 60 they came in a few fords and are great axles  

make sure your axles are the same width as rear. Dana 60 has acouple different width's. Also make sure its a dana 60 not dana 50. Look for casting # on lower part of casting below diff cover.  

Is this a 4x4 or 2WD?

If 4WD, then as the others said, look for a Dana 60 axle out of a '86-'97 F-350, it's completely a bolt-in swap 100% on that truck.

If you're 2WD, then I'd stick some spacers (or washers) under the lower coil spring seats. That would get the I-beam frontend back up to height (unless you're wanting to convert to 4WD, then see above).  

wyomingtrailheader:
make sure your axles are the same width as rear. Dana 60 has acouple different width's. Also make sure its a dana 60 not dana 50. Look for casting # on lower part of casting below diff cover.

Nothing wrong with a D50 if you find one as they are a good bit stronger than a 44 and are "rated" at 90% of torque capacity of a D60 and 10% is not a lot. If you start breaking 50's, you really need more than a 60 too.  

yea i really don't see anything wrong with a 50 its pretty strong, if they are used under modern 250s with tourque monster diesels and big tires im sure theyll be fine for you but if you find a 60 at a good price then go for it.