the sensors need 10-30 vdc to work with 4-20 amp outputs
That's milliamps, right? 4-20mA is an industrial standard, but I'm not up-to-speed on details. You'll need to convert that current to a voltage. I'd guess you can do that with resistors, but you might need an op-amp circuit of some kind. I suggest you do some research on that if you don't get an answer here.
[u]Here[/u] are some over-voltage protection circuits.
The TA2020 is an audio power amplifier designed to drive speakers (4 or 8 Ohms)... It's not right for this application. The Arduino's analog inputs are very high impedance (100 mehohms) so they require (almost) no current or "power". If your signal is below 5V you may need some voltage amplification but you don't need a power amplifier. (And, it's possible that you need some signal amplification even if your sensors are running on 9V.
but the analogread results are still very erratic.
Your sensors are probably built-around strain gauges. These things put-out a tiny voltage which makes it difficult to get a good signal-to-noise ratio. Some [u]smoothing[/u] may help, or you may need something "different" on the analog side. An amplifier amplifies any existing signal and noise together and the active electronics will actually add some noise, so it can't improve the signal-to-noise ratio.