Yes, Jacksonville's Walk Score rating is negatively impacted by consolidation:
^City of Jacksonville prior to 1968 consolidation with Duval County. Before consolidation, the City of Jacksonville was 30.2 square miles.
With a 1950 population of 204,517, Jacksonville had a population density of 6,772.
Jacksonville's 2010 census tracts align fairly close with the old city's boundaries. Unfortunately, unlike Miami, our urban core's decline since 1950 has mirrored that of Rust Belt cities like Detroit, Toledo and Cleveland. Using the census data from the tracts that made up the old city, we can identify the population and density loss of the old city of Jacksonville.
1950 Old City Population Density: 204,517 / 30.2 square miles = 6,772
2010 Old City Census Tract Density: 104,047 / 30.2 square miles = 3,445
Net Urban Core Density Loss (1950 - 2010): - 3,327 residents per mile
Net Urban Core Population Loss (1950 - 2010): -100,470 residents
Nevertheless, when it comes to Walk Score, we'd be looking at a 30.2 square mile city with a population density of 3,445. Cities close to this size today include:
Flint, MI: 100,515 / 33.4 square miles = 3,065 residents per mile
Everett, WA: 104,655 / 33.5 square miles = 3,080 residents per mile
Richmond, CA: 106,516 / 30.1 square miles = 3,449 residents per mile
Clearwater, FL: 108,732 / 25.6 square miles = 4,213 residents per mile
Lansing, MI: 113,996 / 36.1 square miles = 3,171 residents per mile
Their Walkscore rankings are:
40 - Flint
45 - Everett
54 - Richmond
41 - Clearwater
40 - Lansing
If I had to make a guess, I'd say the present condition of preconsolidated Jacksonville would fall within the range of these cities. Somewhere in the 40s or 50s, placing it as a "car dependent (most errands require a car)" or a "Somewhat Walkable (Some errands can be accomplished on foot)" city.
Going a step further. If the 1950 population density remained, here's a couple of similar sized communities today:
Glendale, CA: 194,478 / 30.5 square miles = 6,295 residents per mile
BUffalo, NY: 259,384 / 40.4 square miles = 6,471 residents per mile
Pittsburgh, PA: 306,211 / 55.7 square miles = 5,521 residents per mile
Their Walkscore rankings are:
66 - Glendale, CA
65 - Buffalo, NY
60 - Pittsburgh, PA
All of these cities fall within the "Somewhat Walkable (Some errands can be accomplished on foot)" Walkscore category.