Vibration, Rumble at 1500 rpm

These are always hard to narrow down but I'm wondering if anyone else has noticed this on their truck.
At exactly 1500 rpm there's a vibration/rumbling noise while accelerating in first, again in second, and also in third gears. I think it continues in 4th gear, but is less noticeable. If I'm climbing a hill at 1500 rpm, the vibration is constant but will disappear over and under 1500. I first noticed this in 4-HI when the vibration was felt through the floor. It's much less noticable in 2wd. If I put my hand on the t-case shifter, I can feel the vibe come up through the shifter but not so much on the trans shifter. It's almost a harmonic rumbling sound. I just had a new driveshaft installed by a reputable shop and they are having trouble pinpointing this and suspect it may be the transmission input shaft bearings, or more likely a mount.

Anyone else notice this?

Some specs-
Manual transmission, manual transfercase (no pushbutton), 2.5" lift

Thanks!

 

Problem solved! Only took two years, lol but then again it hasn't been many miles... So, I ended up with another vibration around 55-60 mph, noticeable in 5th gear mainly on level surfaces with minimal foot on the gas. Took it back to the shop and they found some excessive tolerances in the rear slip yoke so they replaced that. Vibrations still persisted, so took it back again, and they found that the rear u-joint looked to be at fault, and rear wheel seals were starting to leak due to worn bearings. Both were replaced and all vibrations previously mentioned have disappeared. I think the wheel bearings were the main culprit because some of the noises were noticeable when I swapped in the old worn out driveshaft to try to pin down the noise. Kudos to Automotive Drivetrain in Tualatin, they went above and beyond with their warranty, and took care of the issue. I hear everything, and those types of things are hard to pin down.