I bought a truck with new virgin tires on the front/rear axle and worn tires on rear

You say "my shop guy". Sounds like someone you trust. Why are you second guessing him now? Let's see, the cost of 4 new tires vs. 4 new tires & a lot of mechanical work. Hmmm....

 

The inter-axle differential will receive the excess wear, not the drive axles.

 

We did this many times when buying ten new tires at once was not affordable.
The rears run on two different shafts.
The troubles start when the tires on a single axle or set of duals are different sizes, even then mostly just the smaller of the two blowing. If all 4 are the same no worries.
Ran many miles like this while saving money for the other 4 tires.

 

The average circumference of tires on the first axle should be approximately equal to the average circumference of tires on the second axle. Rather than mix and match to make this happen, wouldn't it be smarter to trade the older tires for four new ones?

Without matching both axles, the front drive will probably run warmer. Of course, it usually does run a bit warmer because of the inter axle. Heat causes wear. Why take a chance? Someone will buy those used tires.

 

I know a lot of guys that do this with no issue.

I have also heard that it can lead to issues. Personally I just rotate tires between 150,000 and 200,000 miles.

 

If you are driving around with the "differential" locked then yes... Damage could occur. Otherwise there is a reason why it's called a differential! That reason is because it is designed to accommodate the differences in rotating speed that occur during cornering or when tire sets are not evenly worn.