No upshift is typically caused by a faulty kickdown switch (under the "gas" pedal) or solenoid (on the right rear of the transmission). The easiest way to check the switch is to remove the fuse (I think it's either 5 or 8, but check the decoder map for "AT kickdown"). If that doesn't "fix" it, the itself solenoid may be mechanically sticking. You can service it from under the car, without even removing the pan.

Head/gasket has been discussed.

Engine oil leaks: cooler lines (plenty of threads), oil pan gasket (PITA), rear seal (PITA), front seal (not so bad), pan extension (on left side, easy).

Transmission leaks: many O-rings are accessible from the "outside" without removing the transmission: governor cover, B2 and B1 piston and reaction valve, etc. Shift shaft. They will need some hunting to ID which is the troublemaker.

__________________
James Marriott
2003 Buick Regal
1983 300D (228k, frau Auto)
1996 Suburban K2500 (192k, 6.5 turbo diesel/4WD towmaster 10,000)
www.engineeringworks.biz
1987 300SDL junker 170k
1982 300SD junker, 265k