Where is u-joint located and how can I check it?

check fluids ---possibilities --trans fluid low -trans needs filter change-- failing trans--abs problem---if equipped 4wd problem --module -transfer case

if fluid was to thick --probably wrong fluid was in their - possible motor oil - have trans flushed and filter change and replace fluid

just as precaution, have you been changing the trans fluid as recommended. Start off simple, check the fluid level, change the fluid and filter if due. Read the owner's manual for how to check the fluid level. Check for any loose wiring connectors that go to the trans. IF this is no help. Get to a reputable trans shop.

Problem is with the transmission fluid. Get it changed and get the oil filter changed if required. Get the conditions of the gears checked. Those jerks may be due to broken or damaged teeth.

I think he's referring to a gear inside the transmission, part of a planetary gear set. The symptoms point twards a transmission problem, a mount cannot cause this.

inside the gas tank, with the sending uit

you probably should change the transmission fluid and filter with that many miles on it.

This is common problems to the transmission when the fluid is getting to dirty and the transmission fluid and filter needs to be replaced. The transmission fluid and filter should be replaced every 45,000 miles to ensure there is no dirt causing the shift servos from binding. The other thing to is that when the transmission fluid get's to dirty, the fluid gets thicker. It retains fluid retention which can cause conflicts in the transmission sensors. What your experiencing is, your transmission is not shifting back to first gear and sticking in to 2nd gear. When you get on the gas peddle, the transmission sensors realize that it needs to be in 1st gear, so as the engine's RPM goes up and the transmission drops back to 1st, you get the jerking condition which will in turn cause you a $2,300.00 transmission replacement. For now when starting off from a stop, let the engine creep forward on it's own first to ensure that the transmission has shifted in to 1st gear. Then get on the gas peddle lightly till you have the transmission serviced. $100.00 transmission service is better then $2,300.00 transmission replacement. Good luck and do not have a transmission fluid flush, just filter and fluid replacement.

check trans fluid level, change trans fluid and filter, check for broken or loose connections on tranny, check speed sensor on trans,

check fluid level first, run car for at least a minute with the the trans in the park position.
turn off car and check the fluid level within 10 seconds, top up if necessary.
The fluid could also be burnt and the filter inside the trans could also be blocked, otherwise your trans is on its last legs.
Filter replacement is a job for an experiened mechanic, The trans sump needs to come off.
The trans also contains electronics of which contaminated fluid could also pose a problem.