I think there are three possibilities.
1. There is a counterbalance valve built into the tilt valve spool. Could be broke, scored or contaminated. The spring could also be broke.
2. The tilt spool is damaged.
3. Once of the spool centering springs is broke.

1&2 and the top spring can be easily done in the truck. But I can't remember how easy it is to get to the bottom spring with the valve in the truck.

ISZ