09 TRD offroad..weird vibe issue the last couple days

So, here the last few days I have been getting a weird issue on my 09 DC OR. For those who dont know, I have 37's still on stock gearing. I dont do any crazy off roading at this time and still havent got the upgrade on the aftermarket gears yet. 37's have been on the vehicle for about a year. Now here is the issue,

I have noticed lately that around 40mph on my speedo (probably 50 in real life) my truck will be in I think 3rd gear or 4th and around 1200 rpm and gets a weird vibe that shakes the whole truck. This has only happened the past few days. You can stay on the throttle and barely press and it likes the RPM never climbs...if you mash the throttle it will downgear and then rpm will climb...it only happens every so often.

I do tow my boat every now and then. I am starting to wonder if I maybe need to do some fluid changes.