THUMP felt on acceleration

[danielbeek]

Hi All,
Posted this on NPORA also, but maybe best to check locally given it's a Jap import...

Have a very long standing issue that I'd like to resolve - hope you can give me some pointers.
More often than not, when I take off from either stand-still, or simply having slowed for traffic lights (in 2WD), there's a delayed 'thump' felt coming from the driveline. Seems more noticeable when already moving, maybe due to not using clutch.
It's not audible as far as I can tell (over slightly noisy intake, exhaust, and tyres), but it's very clearly felt 'in the seat'.
It only happens once on acceleration, within a short delay of punching the gas pedal. ie car first starts moving, then it happens. I'm pretty certain it's not just lash in the gearbox/diff therefore.
If I slow down really gently, on the gears or just coasting, and then accelerate again, it's often not felt.
It almost feels like the clutch slips a turn, then bites aggressively... But clutch seems pretty good. Not slipping in 5th, up hill, etc... Doesn't hardly slip when dumped with revs.
I did wonder if sticking of the slip joint in the main driveshaft could cause this? ie torque from motor makes it adjust length slightly, but it's binding up.

Vehicle details:
R3M Urban - LBYD21 (thats a TD27T diesel model)
31" ATs
5 speed manual
H233 read diff with LSD (fitted from a higher spec vehicle years ago)
2" raised rear springs, and wound up torsion bars (no panhard drop bracket, or longer swaybar links)
Good HD shocks (issue predates replacement anyway)

Thanks for any suggestions!

[zukmeista]

Check all the bushes in the rear end

[Fwb2700]

Driveshaft universal maybe

[kbushnz]

Does it run a Dual mass flywheel. They can fail and cause movement in the clutch area... Personally never felt what one is like when it fails or is about to fail..

[MihiT]

I know you discount backlash, but check it. Jack the back, 4th gear, have your apprentice turn it forward, then back. Look for slop in the uni's, loose hangers, if it's in diff then it'll have to be felt through the hub.

[danielbeek]

thanks heaps for all the suggestions guys, hopefully go through some of these next week.