Problems after pulling boat surging while in R

[Lifsvr7]

OK... I pulled the boat today, little farther than normal... About 50 miles round trip- Boat weighs about 6500 lbs. (light compared to most ppl tows) When I got home & backed it up drive, I noticed that it was surging a little and had a little difficulty, so instead of beating on it I just shifted it in 4 low. Went right up drive no prob. (driveway is by no means steep) So today, I backed out of a spot, and noticed that when I put it in R, it takes a sec (normal) then while its in R I feel a slight jerking.... SO I sit there waiting, it never goes away, kinda like its slippin in and out. I back out, slowly and still jerking. So, thinking that maybe the truck got used to pulling the weight, I reinstalled the tune...(I.D.'s xtreme race) Drive it a short time giving it time to relearn, noticing that it is slacking around 40 mph. So when I get home I put it into R again doing the same thing... Now here is the thing too.. truck has 44000miles, and trans was redone at 36000 miles. Wondering if it really is the trans or what... Fluid is NOT burnt and is at level. Any help and insight would be great...

ps truck set-up in sig