that he was a top tier back in the league so I don't think he is underrated at least by Rams fans. I will say that you rank him higher than I do. I always though he was kind of like the top of tier 2 or bottom of tier 1. Take 2006 for example, one of his better yards per carry season after his rookie season. His 4.4 put him 9th out of the 1000 yard runners that season. If you go by rushing yards instead of yards per carry, he placed 5th behind 1) Tomlinson 2) Johnson 3) Gore 4) Barber 5) Jackson
And that was one of his better seasons.
It is really tough to evaluate Jackson as the bad was bad around him. I don't know if that helped him because the entire offense gravitated to him or hurt him because he didn't have enough around him to keep the defenses honest. His best yards per carry came with Martz coaching him very early in the season in his rookie season at 5.0 yards per carry where there was some diversity in the offense, but he had to share with Marshall Faulk on the tail end of his career.
He put up 8 1000 yard seasons in a row so that is impressive consistency.
Hence, I feel at least most Rams fans feel he was a top tier runningback during his prime and that is what he was a top tier. Typically between 5-10. His career stats have him 18th of all time rushing yards and that is impressive so he had longevity as well. However, his yards per carry career total at 4.1 only has him as 147th of all-time.