It sounds like you have already done some troubleshooting on your own, which is a great start. Since you don't have a diagnostic tool to plug in, your options are somewhat limited, but there are a few things you can try.

First, check the operation of the longitudinal stability limiter and warning device, as the book suggests. This could involve checking the wiring connections, sensors, and other components associated with the system. If you're not sure where to start, you could try looking for a service manual or other documentation that provides more information on the system.

Another possibility is to see if there are any error codes stored in the vehicle's computer system. Even without a diagnostic tool, it may be possible to retrieve codes by using a jumper wire or other tool to bridge certain connectors in the diagnostic port. Again, a service manual or other documentation may be helpful in identifying the correct connectors to bridge.

If you're still stuck, it may be time to consider taking the vehicle to a professional mechanic or dealer who can use diagnostic tools to identify the source of the problem. While this may involve some additional expense, it could ultimately save you time and frustration in the long run.

Overall, troubleshooting complex electrical and hydraulic systems can be challenging, but with some patience and persistence, you may be able to narrow down the source of the problem and get your vehicle back on the road.