Transmission Buick

'86 Lesabre fwd came with 440-T4(4T60) 4 speed automatic
'90 Skylark fwd usually had 125C(3T40) 3 speed automatic
swapping them would not be advisable

If you need transaxle for Lesabre, you could use any 440T4 from '85-'91 fwd GM vehicles provided that chain sprockets and final drive ratios are compatible,also minor electrical changes to valve body harness.