97 Mazda B4000 truck, 4.0L V-6, 4WD, 5-speed.

Bought the truck in January, used with 89,000 miles.  After driving it
for a month or so, I found that after driving it for 20-25 miles, the
transmission almost would not shift into 1st or 2nd --  I had to shove
so hard on the stick I was afraid of breaking the stick.
Double-clutching didn't help.

I bought an extended warranty when I bought the truck so the dealer put
in a new (rebuilt?) transmission.

Now, it's doing the same thing in 2nd gear -- drive 20 miles or so and
it becomes very difficult to shift into 2nd, either from 1st or 3rd,
although downshifting is much more difficult.  Double-clutching helps a
lot.

I suspect a bad synchro gear and it's only going to get worse.

I will check my repair ticket to see if the tranny they installed is
warranteed before I go back to the dealer.

Meanwhile, does anyone have any suggestions?  Is this a common problem
with this truck and its Ranger twin?

Thanks.