Engine/Transmission (1998.5 - 2002) Vibration

[hcpBILL]

When pulling a load I get a slight vibration at about 1800 RPM when accelerating through 2nd & 3rd gears. It is not severe but is enough to be noticable. There are no other signs of problems. The clutch does not slip. Any ideas for what this might be? No engine modifications except for the 4" Jardine and two mufflers.

Thanks

Bill