Im replacing the pinion seal and bearings on my dana 90 rearend and i need to know what size socket i need for the bolt to take out the pinion assembly. The rearend is in my 72 f250

 

A Dana 90? Never heard of it. Your truck came with a Dana 60 rear, and a 44 front of its 4wd.

 

Should be a D60 in an F250. The pinion asembly doesn't come out without a complete teardown, but the yoke can be pulled off fairly easily with (IIRC) a 1 1/8" socket. I have had torque to remove range from just past finger tight to never giving up.
However, it is not very common for a seal to just leak on it's own, it is usually caused by either a loose pinion bearing, or a plugged vent tube.
Extreme care must be taken in doing this job, though- Block both sides of a front tire, and put the truck in neutral. Rolling away sucks when you are under the truck. Before you remove the u-joint straps, check the pinion yoke for play. If you can move the yoke at all then the axle will need to be rebuilt. There is a non-serviceable crush sleeve on the pinion that sets the bearing preload, with a loose preload then the gear pattern can't be corrected and the gears will need to be replaced. At this point if you can't do it yourself, it'll be cheaper to replace the axle with a decent used one.

 

I found a few 205 TFC with not more then 20-30lbs TQ on the out put yokes that the seal leaked at PnP kind of odd looked like they've never been removed before. May be put together on a Monday.
Orich