Time to replace them.... possibly Bilsteins. any opinions/experience? recommendations? Want a decent ride with control when pulling the boat....

I've put Bilstein 5100 on several vehicles, and happy with the results. I currently run them on my F-350 and my 2500HD.

THe 4600 and 5100 series I think are set up the same, the 5100 is longer for slightlifts?? anyways, not overly stiff or anything?

Definitely not.
On my F-350 I went from Edlebrock shocks over to the 5100s and noticed a marked improvement. Note, I did go to a double shock setup on the front.
On my 2500HD, I went from stock shocks with 100K miles on them to the 5100s, and whoah, it really helped tame the ride.
I think if you went to the HD series, shocks, that's when you would see stiff and too much resistance for a DD as well.

When I was a mechanic we used a lot of KYB shocks. I always liked them better than the Monroe Little firmer ride than OEM due to increased dampening, not harsh or anything, just would settle any harmonics extremely quick. I used to put the KYB Mono-max on my 3/4 tons and they worked real well towing. Also used to beat the truck on the sand dunes and never had one break unlike the stock GM.