New 2nd Gen Owner

For as long as I can remember I have wanted a Taco, about 25 years or so. I finally did it! Only issue is I believe the rear differential is smoked... I got it for a good price but now I need to know if I am diagnosing it correctly. There are many sounds I am hearing and its in the rear of the truck. Let me start with telling you all its a 2009 PreRunner SR5 TRD Sport 5ft bed and its RWD. Now back to the sounds. Its making a loud grinding sound and clunking when I press the gas. When I let off the gas it calms. When at a stand still in park and I put the gear in D, It makes a loud clicking sound and it sounds like its coming either from the rear U Joint on the Drive Shaft or the inside the case of the Rear Diff. If I switch to R it clicks LOUD and then I put it back in Drive it clicks again. I found a whole used rear diff from a 2010 with 101k miles on it for around $600 but don't want to spend the money unless i know for sure that its the issue 100% what can you all advise. I would appreciate any advise or options. Also how do I find out the Ratio that my truck has?