I can't see where a set of 215/70R15 tires would be so different from the factory size tires on that truck to cause that kind of highway RPM. From a (195/70-14") 24.7" tire at the smallest factory sizing, to what he's running (215/70-15") being 26.9" - That an 8.9% difference in diameter. About 60 less revolutions per mile.
In fact, him going with larger tires than stock would drop his highway cruising RPM. Even with his size of tire (a 27" tall tire) and guessing that he could have a 4.56 rear gear, he should still only be turning around 2500rpm or so at 55mph. A 3.73 rear gear would have it turning around 2000rpm at 55mph. Even with a 4.56 rear gear and his 215/70-15" at 65mph his engine speed in OD/5th should still be around 2900rpm.
The OP mentioning the truck going through the gears really fast without much change in speed, and the high revs on the highway, leads one to think it's got some extremely low gearing in the rear axle.
Tire Rack and several other sites list the following as available factory/optional sizes on his 88 2wd:
1.
195/70R14
2.
205/70R14
3.
215/70R14
Of course, with tire size differences, this could potentially add to the confusion, as it will cause the speedometer to be off. So he thinks he's running 65mph and he's not.
Makes me wonder what the true engine RPM really is at highway speed. Also makes me curious about the clutch..