My 93 Chev Lumina Z34 tranny has odd shifting

This doesn't sound good. The first thing I would do is check the fluid level, it my be low, this would cause slipping. If the fluid level is correct inspect its color and check for water on the stick. Your cooler my have a leak causing water to leak in the fluid causing the clutches to slip. If this has happened have the system flushed and the radiator repaired. Make sure to add a pint of Lucas transmission fix. This will help the water damage to the clutches. If these steps do not work, the problem needs to be addressed by a transmission specailist. The sooner the better.
I wouldn't waste my money replacing the modulator pin, if it didn't shift, or shifted poorly without the slipping I might give it a look along with the vacuum lines. Hope some of this helped.