I have a 90 w 250(4x4) with a cummins and last week and my rear axle came out and ruined the outer housing where the big nut goes on,I could not find a rear end with the 307 gear ratio(very rare) that it came with,I can get a rearend out of a 85 dually that has a 456 ratio and have one side of the axle housing taken off and put on the old rear end,does anyone know if that will work or any other kind of fix that might work?

 

If you can find a complete rear axle asm, I assume it's a DANA/Spicer 70? just have the whole axle replaced and then have the shop install a new ring and pinion with 3.07 ratio. Sounds like you may have to weigh the costs of labor vs the cost of parts. Call around for a price on a new ring and pinion. Perhaps you could find the axle housing only and have your old parts installed?
Search www.car-part.com or put out a request on www.junkyarddog.com
www.ringpinion.com
www.drivetrainindustries.com these guys have been in business for 60 years
They may want your BOM (Bill of Material) number off your old rear end.