1996 Toyota Tacoma, 4X4 5 speed 6 cyl. club cab...When moving there is a vibration, even when cruise is on. When cruise and accelerator let up, as in going down hill. the vibration stops. There is a slight sound, not really a clunk when gears are changed. Someone suggested transfer case, I'm wondering u joints?

Clean your water trap on diesel filter

I would look at the u-joints before looking at the Tcase. Lift the front end (both wheels) off the ground at same time, crawl under and shake the front drive shaft. up down side to side. If it's tight the let front end down and get the rear end up in the air (both wheels) off the ground and do the same this to the main drive shaft. If the rear U-joints are failing (closest to the read Diff) take a real good look at the Diff. Make sure it's not leaking fluid, If it is you may have a bad pinion starting to go out also.

I think you have an engine wiring harness issue going on here.

Yes it sounds like its time for a tune-up plugs,wires,fuel filter and air filter

The port is located under the dash on the drivers side. It is located to the right of the steering column. Should be black and pointing downward.

Are the new springs that were fitted original factory or after market?
New springs will be less flexible than old ones until they settle into use. This will explain the increase in ride height. Old springs soften so the ride height drops a small amount. At higher speeds your getting more down force so the suspension will settle.
Remember she's a 20 year old truck, things start to sag as you get older.

I think its your gearbox as what you've saying is that you were drive with heavy moving trailer across the country and in the mountains it happiened i believe its friction rings in your gearbox what you've done calls RAPE GEARBOX i'll explane to you why

what is friction rings its a metal made rings inside your gearbox that are spinning. every time you switch your speed u said u've got 6-speed manual transmittion which means there are 12 friction rings on each of the speed + rear speed (R) now what happiens when you going up hill oil cerculation going faster as frictions spin faster but if you have not enough oil comming into the frictions then they start to rub on each other and they die thats maybewhy u have vibration

This is just my opinion

HI, i have a 96 toyota tacoma sr5 the problem is the catalytic converter my suggestion is to take it to an exhaust shop and have them run a pressure test on your exhaust.

You may need to have tyres balanced and wheel alignment done.

check your vss on your tranny