Tach isn't working right

So my tach seems to be higher sometimes when i'm driving the vehicle. Like in fifth gear it's around 2300 rpms at 70 and sometimes it's at 1900 rpms at 70. Can't seem to find a reason it would do this. I was just wondering where the tach gets it's readings from and if I should suspect that or the speedo being wrong. I have a 95 xj with a 3in lift and 31's on a stock 3.07 gear and ax-15 and 6 cyl. I've already replaced the tps. just not sure where to go with troubleshooting this. Thanks in advance for any help.

bump for no ideas?

My speedometer behaved like this when the sending unit was going out but I don't know about the tach.
dn

crank position sensor is the original signal source for the tach...not sure if it gets modified by PCM before feeding the guage..

Well i'll just save up and get a new cps then and see if that helps resolve the issue.

Deleted