Transmission Problem - Jerks Into Reverse

About all you can do your self is check the fluid level. If the level is OK and the fluid looks OK on your 2000 Buick LeSabre the next step would be to scan the Power Train Control module for codes, the transmission pressure control solenoid should also be checked for proper operation.

Could be a bad Transmission mount.

if it makes a clunk it may be your u joints easy fix.