1991 Blazer 4.3 - Tire question

I know what size tire is factory on this truck, but the guy I just bought this one from had several different size tries on it and I wondered, the back tires are sizes 235/75-15 and 215/70-15, does that make for any strain on the rear axle or gear? It is a 2wd. Just want to make sure the axle isn't binding in any way because of the conflicting dpr's.

Also, both of those tire sizes change the speedo accuracy but how do I know which one to go by? Would it split the difference? The smaller tire is almost correct in speed while the other is increased over the speedo, just not sure how to tell how fast I'm actually going and can't replace tires for another week or so.

And any other notable points about the 91 4dr 2wd would be helpful too, in case some random thought pops in your head while reading this ;)

Thanks.

My 91 Jimmy 2Dr has a tag on the door jam that says to use 205/75 R 15