I'm wanting to swap the rearend in my f-body. I think that I have a 7 1/4 and i want at least an 8 1/4. I'm not sure what to pull it from.

Diplomat, Gr. fury (cop cars!) are a direct bolt in and they made em all the way to '89! A 5th ave is also a direct swap but most of those I see have the same crappy 7-1/4 that you are trying to get away from. 80-83 Mirada or Cordoba also direct swap. If you get to a JY, take a tape measure and check Cherokees and Dakotas (although if you go Dakota youll have to stay between 87-90 unless you want 6 lug axles!) These would at a minimum need the perches cut off ane rewelded.

Most of the M-bodie cars came with a 2.25 rear axle ratio. A few cop cars got 2.93's.

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82 D150-225/727
02 Dakota-3.9/5 speed
87 GMC C7000-8.2 Detroit Diesel/5+2

A cop car will almost always have the 8.25 axle in 2.9 ratio, and often with sure grip. Will also have bigger brakes, rear sway bar, heavy duty springs and rear shackles. Wagons almost always have the 8.25, usually in 2.4 past 1980. I have seen a 2.2 in a wagon, but that is not common. If you find a cop car, get the front sway bar as well, as it is bigger, 1.125 vice 1.0 for most F, M, J body cars. Performance suspension sells the bushing kits to mount the bigger bar.
The 2.2 ratio started around 1980, same year that the 904 tranny went to the lower first and second gears.

This what i did i put a 8-3/4 out of a b body in my aspen, had to grind the spring perch loose move them in about 1/2 inch and set the pinion angle make sure that it is centered then weld it together.