D12 member Proof's debut album "Searchin 4 Jerry Garcia" is planned to drop after D12's second album, possibly sometime in 2004. This is the way things will drop according to www.aftermathmusic.com :

Obie Trice(Sept 16th), G-Unit(November) D12(November/December) Proof (December/January) 50 Cent (February/March 2004) Eminem (May/June 2004)

There is no release dates for Dr. Dre: Detox and Ice Cube's Aftermath debut but expect them both to drop within the first half of 2004.

Source: Fiendworld.com