OK, so here's the update.

After purchasing the 1998 blower motor housing from eBay for $100, I thought I'd check the relay and see what kind of condition it was in. It looks like the original Denso relay to me, very crusty with some carbon scoring around the terminals. I decided to change the relay out before going through the entire process of disassembly on the blower housing in the car. I replaced the relay with a new Denso relay, and it seemed to do the trick. However, summer is coming, and the blower had problems when it got hot outside, so I'm going to hang on to the 98 blower assembly for the near future. We'll see how well this fix works in August. My suggestion to anybody having this problem would be to check the relay first. My experience with relays is that they usually just fail, but I'm thinking that the conductors inside the relay have degraded to the point where the motor isn't getting it's full 14.7 volts, especially on a hot sunny day under the hood painted with 14 coats of Mr. Toyoda's finest tuxedo black poly.