90 FWD 22RE kicks out of gear in 1st & 2nd unless I hold the stick in.

New/rebuilt transmission

FWD? Ya mean 4WD w/5spd? My '90 4Runner did same thing. I took off the shift lever and replaced a dark green nylon or plastic washer looking thing - shift lever seat ($10 part at dealer) and replaced old trans oil with synthetic and never did it again. See link-

Shift lever pilot bushing # 33548-31010 (green)=3D$7 CDN Shift lever seat # 33505-35020 or 33505-35030 (white)=3D$16 CDN

Note: the 2 above #'s fit all trannies:G,R,W series (22R,R-E,3VZE)84-95.5,no need for entire shift lever retainer (85-95.5 pre-Tacoma)

Shift lever pilot bushing # 33548-31010 (green)=3D$7 CDN Shift lever seat # 33505-35020 or 33505-35030 (white)=3D$16 CDN

Note: the 2 above #'s fit all trannies:G,R,W series (22R,R-E,3VZE)84-95.5,no need for entire shift lever retainer (85-95.5 pre-Tacoma)

As a few have recommended, the shifter seat may be worn and/or destroyed. It is a very easy/quick fix. I would recommend getting the replacement seat and socket from Marlin, their two replacements are much better than stock. Worked great in my truck. They are extremely knowledgeable about these trucks, if you have any questions about which tranny you have or how to install these, give them a ring.