The most recent (AFAIK) data from 2014 U.S. Religious Landscape Study are really awful.
http://www.pewforum.org/religious-landscape-study/religious-tradition/jehovahs-witness/
On other hand, the margin of error is nearly +/- 8.5 percentage points (Report, p. 93, 96).
As to the question, it was:
Last year, that is in 2013, what was your total family income from all sources, before taxes? (Report, p. 195)