I don't believe there is a speed limit in park, but those vintage of trucks had a speed limiter of 98mph.
Depending on what rear end, tire size, transmission, and sender you have (compared to the stock one from the 89), the ECU might be triggering that.
Can you try discoonnecting the speedo sending unit at the transmission to see if that affects it?
If so, there might be some ways to work around it.
