I don't think Census pages are missing because I can't find them on the 1870 Census in Ancestry and FamilySearch. You can also look for them in www.censusfinder.com
I think Elvira also died in 1860's and the family split up.
James M. Rankin Jr. b, abt. 1810, Alabama & d. 13 Oct 1869, Brazos Co., TX
Source: Rankin Family Tree, ancestry, & Elvira's pension claim Waller Co., TX, June 1877
The Grimes County Records are on microfilm at the Sam Houston State University, www.tsl.state.tx.us/arc/local/grimes.html
. Your local public library can request the microfilm via inter-library loan.
The Navasota Public Library, www.navasotatx.gov/p/getconnected/288
has genealogical resources.
Also the Library of Congress has digitized newspapers for free 1836-1922, http://chroniclingamerican.loc.gov/search/titles/results/?st...
which might include Grimes County. Successful Searching!