Hmmm, you need to know more what is being quoted. The front diff is co-located with the transaxle and is bathed by the transmission fluid. If the $175 includes the front diff, which would involve about 4 gallons of ATF WS fluid (simple drain and refill), that price is okay. But most likely they are just going to change the rear diff and transfer case only, which only take about a pint of fluid each and would be about a 30-minute job. If so, $175 is pretty steep, IMO. My dealer told me they were going to do a 'complete differential service' and neglected to tell me the front diff was not included.
The only hard part of the rear diff and transfer case service is getting the RAV off the ground for ease of working under there. If they already have it on a lift for an oil change, they are making easy money to charge so much to change a couple pints.