Engine not kicking down after driving.

I just swapped in a used 4.0L engine into my 88 XJ and I'm in the process of working out all the bugs.

The problem I'm having right now is with the engine revving high after I put in the clutch. If I am driving at say 2k rpm, put in the clutch and let off the gas the engine doesn't kick down. It is staying at that high rpm. If I give the gas a quick blip it will usually kick down but it won't on it's own.

Any ideas?

Sticky throttle cable or linkage. Or possibly a worn throttle body