Last week while driving my pickup on the highway here in Wichita in a rainstorm, I was going between 70-75 with the pedal to the floor to pass an 18-wheeler (probably 2 mins of full throttle) in 5th gear. Without any warning my truck lurched or jolted and there was a thunking noise from somewhere in the drivetrain. As I passed the truck and slowed down to 65-70 it lurched one more time seemingly randomly.

Does anyone have any idea what this could be.

Additional information: 1985 Toyota Pickup 4x4 dlx long bed 22R 5speed 130k
All stock except: LCE Timing cover kit, Redline transmission fluid. 1" wheel spacers, 2.5" spring lift and 31" tires on stock gears. (originally had 27" tires)
I replaced the clutch last year with a brand-new Aisin kit because it was slipping when I bought it. I have noticed slight oil residue covering the bottom of the transmission and transfer case that makes me believe there might be an engine oil leak around the rear seal but not enough that it drips on the ground.

My first thought is that the clutch slipped and then regrabbed while I was trying to accelerate but it didn't feel the same as when the clutch used to slip when I bought it. Before I replaced the clutch, at high revs or high load the clutch would seem to let go and not reengage causing the rpms to shoot up while the truck didn't move forward until I let the rpms or the load come back down.

This new incident on the highway did not feel the same. It was a quick violent jolt and then the truck resumed normal operation without any intervention from me. I've considered that maybe my tires just broke loose for a second in the rain, but I didn't experience any fishtailing, and the noise/clunk seemed to originate around the transmission.

I tested the truck again this morning on the same highway without rain and I was not able to duplicate the issue. Does anyone have any ideas?

 

Sounds like you hydroplaned a bit.

 

I hope that's all it is. I've hydroplaned before in this truck but it never felt like a huge jolt with a thunk afterwards.