You're right, the tax rate for Oklahoma was unchanged from 2017 to 2018, the top rate is 5%:
Oklahoma tax rates for single filers or married filing separately: https://www.ok.gov/tax/Individuals/Income_Tax/Filing_Information/General_Information/Single_Taxpayer...
Oklahoma tax rates for joint filers: https://www.ok.gov/tax/Individuals/Income_Tax/Filing_Information/General_Information/Joint_Filing_Hu...
It looks like you are married filing jointly based on your 2018 tax return based on the rates below ((taxable income of $85,197 - $12,200)x5% + $255). So that 2018 tax amount looks correct.
I'm unable to re-calculate your tax for 2017, I'm calculating it if you were single as $3,877.50 ((taxable income of $81,321 - $7,200)x5% + $171.50) or if you were married filing joint as $3,711 ((taxable income of $81,321 - $12,200)x5% + $255).
Can you double check the 2017 tax total?
Is there a credit or some other adjustment that you see that got you to the $2,174?
