Extreme “shake” towing uphill in 1st.

Searched this issue for hours with no real results…. When I’m climbing a steep hill in 1st gear I get a shake on the whole truck. It’s a very rhythmic shake almost like the driveshaft is out of line with the trans. That’s about the only way I can explain it. It’s even worse if I’m towing my small utility trailer. It only happens when I’m in 1st gear climbing a hill. I know I’m not lugging the motor etc. I installed all new u joints and carrier bearing about 2 years ago so I wouldn’t believe that to be an issue but what do I know. It happened yesterday when pulling a fairly heavy trailer load of firewood up a steep paved driveway and was so bad I was expecting to hear some really expensive noises by the time I topped out. I’ve owned this truck since it had 115k on it and it now has 160k. So about 10 years. I wouldn’t trade it for a brand new one! It’s a 2006 double cab TRD Off Road with the 6 speed manual. I’m thinking it could possibly be motor/trans mount related? Sorry for this post being all over the place…. Trying to type this out when I get free time at work…

 

Axle wrap with the stock floppy noodle leaf springs. Its a "character trait".

I bet it does it as well if you are unloaded on flat ground but take off from a dead stop quickly.

 

Did you (whoever) phase the 2 peice properly? No china ctap replacement parts? Trans fluid condition/level?