The very first use of the Suburban name was by Dodge, in 1933, for the "Westchester Semi-Sedan Suburban", a truck based station wagon. Plymouth started a year latter, as did Chevy. The name appears to be more descriptive of wagons in the 30's and 40's, used by Chevy, Plymouth, Dodge, Nash and Hudson among others. Plymouth and Chevy kept using it the longest. Plymouth appeared to flip flop on use, sometimes the wagons were separate Suburban models, sometimes not. My experience is this is some brand/ marketing or advertising manager trying to emphasize what they thought was important. Plymouth stopped using the Suburban name ("abandoned") in the late 70's (with the demise of the C bodies?) and that allowed Chevrolet to register Suburban as an exclusive trademark only in 1988(!).
BTW, could you post a picture of the tailgate on your wagon? I am looking to see if the 65/66 Plymouth gate is the same (except for trim) as my 66 Town and Country's rusty and hard to find one.
Thanks,
Mark