“‘Front module designs [were] completed, and production-implementation plans [were] affirmed [in 1984].’ ‘In armor technology, the BRL achieved two important technology advances. The first was the development and demonstration of an improved integrated armor for the 1990s; This eventually became known as the M1A1 Heavy.’”
“During Operation Desert Shield/Storm, we constantly heard reports lauding the Abrams MBTs: the M1, IPM1, M1A1, and the M1A1 heavy armor–the best the world has to offer.”
For context, the “frontal module design” is referring to at the very least the frontal hull armor, as the only specific panel listed was an example of a non-adopted upper glacis (UFP) design: “In 1980, ‘[An] upper glacis design [was] developed and demonstrated, although not adopted.’” This means that the “frontal” modules they were improving include the hull ones on the hull, and are not limited to just the front of the turret. Therefore we can assume that since they did not specify the M1A1 HA only had frontal turret armor, there is no reason to assume that the improvements only occurred on the turret.
Also, the only other versions of the Abrams listed were the base M1, IPM1, M1A1 HA, and M1A2. I have so far not claimed the IPM1 had DU, and from this we can gather than at least the pre-1992 production (base) M1A1’s did not have DU but the M1A1 HA did have DU in the hull. This report doesn’t list anything specific about the M1A2 armor besides a BRL hull armor proposal that would “[provide] 35% more KE protection and 25% more SC protection than the present M1A1 hull armor,” and that “‘the latest Abrams variant, the M1A2, currently undergoing final development is also the beneficiary of BRL armor research.’”
Additionally, we know that some M1A1’s have DU in them going off of this document from Congress (Alternatives for the U.S. Tank Industrial Base): https://www.cbo.gov/sites/default/files/103rd-congress-1993-1994/reports/93doc04_0.pdf
Spoiler
So, as you can see the M1A1 HA has DU in the hull, DU-hulled Abrams tanks have been used in combat during Operation Desert Shield/Storm, some M1A1 tanks have DU armor in them (all M1A1s past the initial 2,266), and that the M1A2 also have DU in them (although not specified where in the sources provided in this comment).