Welcome to iRV2.
Sounds like you might have a speed sensor problem on the transmission.
Do you have any issues with the trans shifting at the same time ; harsh shifting ? Or are the only systems in trouble speedo & cruise?
Do you have a touch pad shifter control?
Have you checked for any fault codes in the trans controller?
Do you happen to know your transmission series ? 3000? 4000?
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.